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Original articleAlcohol consumption among first-time mothers and the riskof preterm birth: a cohort studyMaria T.G. Dale PhD a,*, Leiv S. Bakketeig PhD a, Per Magnus PhD a,ba Department of Genes and Environment, Norwegian Institute of Public Health, Oslo, Norwayb Department of Medicine, University of Oslo, Oslo, Norwaya r t i c l e i n f oArticle history:Received 10 April 2015Accepted 31 August 2015Available online 30 September 2015Keywords:Cohort studyAlcohol consumptionPreterm deliveryPregnancyHealthy drinker effecta b s t r a c tPurpose: Our aim was to explore the association between alcohol consumption, before and duringpregnancy, and the risk of preterm birth among 46,252 primiparous mothers.Methods: We obtained information on alcohol consumption from questionnaire responses at pregnancyweek 15 from the prospective, observational Norwegian Mother and Child Cohort Study. Data on pretermbirth, categorized as delivery before gestation week 37, were retrieved from the Medical Birth Registry ofNorway.Results: Among the participants, 91% consumed alcohol before pregnancy and fewer than 20% reportedconsuming alcohol during pregnancy. The adjusted odds ratio (aOR) for preterm birth associated withprepregnancy alcohol consumption was 0.81 (95% confidence interval [CI], 0.70e0.95). We did not find arisk reduction for overall drinking during pregnancy, aOR ¼ 1.03 (95% CI, 0.90e1.19). However, doseresponseanalyses showed tendencies toward adverse effects when drinking 1e3 times per monthduring the first 15 weeks of pregnancy, aOR ¼ 1.51 (95% CI, 1.14e2.00).Conclusions: We did not find any effects of alcohol consumption during pregnancy, whereas prepregnancydrinking was associated with reduced risk of preterm birth. Residual confounding mayhave influenced the risk estimates, especially before pregnancy, as nondrinkers have lower socioeconomicstatus and well-being than drinkers. 2016 Elsevier Inc. All rights reserved.IntroductionMost public health and medical authorities advise total abstinenceof alcohol both during pregnancy and when contemplatingpregnancy [1,2]. This advice is an invocation of the precautionaryprinciple since existing evidence is inconclusive as to whether thereis a threshold dose below which alcohol intake is safe. Overall, thereare few studies that indicate adverse effects of low levels of alcoholintake [3,4], whereas there is broad consensus in the literature thatabusive and heavy drinking has a teratogenic effect on the fetus,such as the risk of fetal alcohol syndrome, growth restriction, birthdefects, and neurodevelopmental problems [5e7].A concern has been that alcohol intake may increase the risk ofpreterm birth. Preterm birth is among the leading causes of childmortality, to which 35% of neonatal deaths on a global basis areestimated to be attributable [8,9]. In 2013, about 5.8% of all births inNorway were preterm, as defined by theWorld Health Organizationas delivery before 37 weeks (259 days) of gestation. This rate issimilar to the rest of Europe and other developed countries, where5%e9% of all births were preterm [10,11]. From the 1990s, thechances of survival have increased because of better medicalintensive care with advances such as antenatal corticosteroids,assisted ventilation, and administration of surfactant to preventlungs from collapsing [12]. Almost in parallel with these improvements,preterm birth rates appear to increase [13]. Overall, from1990 to 2010, there has been a 19.4% increase in the preterm birthrate in high-income countries (from7.2% to 8.6%). The United Stateshas a particularly high rate (12%) [8]. Better knowledge of causesand mechanisms is needed to prevent preterm birth.Several studies on the subject of alcohol and preterm birth exist,but the findings are inconsistent. Apparently, some studies suggestthat the relation between alcohol and preterm birth is J-shaped orU-shaped. A study using a Danish pregnancy cohort suggested thatan intake of more than seven drinks of alcohol per week increasedthe risk of preterm birth [14]. Another study showed that the risk

Corresponding author. Department of Genes and Environment, Division ofEpidemiology, Norwegian Institute of Public Health, Box 4404, Nydalen, Oslo 0403,Norway. Tel.: þ47-21078106; fax: þ47-21078252.E-mail address: MariaTeresaGronning.Dale@fhi.no (M.T.G. Dale).Contents lists available at ScienceDirectAnnals of Epidemiologyjournal homepage: www.annalsofepidemiology.orghttp://dx.doi.org/10.1016/j.annepidem.2015.08.0131047-2797/ 2016 Elsevier Inc. All rights reserved.Annals of Epidemiology 26 (2016) 275e282increased when more than 10 drinkswere consumed per week [15].However, both studies reported that lower levels of consumptionmight provide a small advantage as the risk of preterm birth wassignificantly decreased compared to nondrinkers (relative risks of0.7e0.9) with an intake of alcohol below four drinks per week.Decreased risk at low to moderate alcohol intake was also found ina cross-sectional study byWright et al. (1998) [16], who reported arelative risk of 0.4 (95% confidence interval [CI], 0.2e0.7) for pretermbirth associated with alcohol intake in the third trimester.Looking only at induced preterm birth, Meis et al. (1997) [17] foundthat alcohol intake reported in gestational week 24 was associatedwith reduced risk (relative risk 0.34, 95% CI, 0.15e0.76). A veryrecent prospective cohort study by Lundsberg et al. (2015) [18]support these earlier findings, reporting reduced risk of pretermbirth with low to moderate alcohol intake both during early andlate pregnancy (relative risk of 0.79 and 0.60).Other studies suggest that daily drinking has an adverse effecton preterm delivery. Jaddoe et al. (2007) [19] found that an averageconsumption of one drink per day either during early or latepregnancy had a relative risk of 2.5 (95% CI, 0.9e6.8) for pretermbirth. Another prospective study showed a dose-response risk forlow and moderate intake (0.10 oz or >0.10e0.25 oz of absolutealcohol per day) during late pregnancy and risk of preterm birth(relative risks of 2.88 and 2.96) [20]. Both studies indicate that dailyalcohol intake has a risk-increasing effect and that greater theexposure the greater the risk, but no studies have found adverseeffects of occasional drinking. A recent systematic reviewand metaanalysisby Patra et al. (2011) [4] sum up the aforementionedfindings: only heavy consumption, 1.5 drinks per day and more,increased the risk of preterm birth, whereas lower levels wereassociated with reduced risk or no risk.These inconsistent findings of risk at relatively high consumptionlevels and no risk and/or reduced risk at moderate and lowlevels may be due to the heterogeneity across studies, large divergencein defining and measuring alcohol consumption, and thatsome findings are not adjusted for important lifestyle and relatedsocioeconomic factors [4,19e21]. Another source of confoundingcould be undetected differences in health-related behavior due toprevious reproductive experience [21] or due to the presence ofpublication bias [3].Most pregnant women in Norway either abstain from alcohol ordrink occasionally. The proportion of women who are occasionaldrinkers may be relatively large, a recent study in Norway indicatesthat one in 10 women maintain light alcohol consumption duringpregnancy [22]. A more solid scientific basis for advice to thesewomen is needed and should be provided by epidemiologic cohortstudies rather than studies focusing only on women with highalcohol consumption [23]. Our objective is to examine critically, in alarge prospective pregnancy cohort, the effect of alcohol before andduring pregnancy on preterm delivery. Studies suggesting that lowlevels of alcohol intake have a protective effect have been thesubject of controversydis there a biological explanation for thisassociation or is the outcome attributed to maternal lifestyle andbackground variables? A major problem in this area of research isconfounding by background factors such as socioeconomic statusand lifestyle. We had the opportunity to control for prepregnancydrinking, as well as the drinking pattern of the spouse. Togetherwith educational level, these two variables reveal a major part ofthe family’s lifestyle that might have confounding effects, whichallows us to identify better the alcohol-specific effects on pregnancy.This present study provides dose-response information inthe drinkers versus nondrinkers analyses, making it possible toexplore if low amounts of alcohol relate to preterm risk. As alcoholintake was assessed in the first trimester, prospective to the birthoutcome, we avoid recall bias and under-reporting among womenwith adverse pregnancy outcomes. By including only singletonsamong primiparous births, we isolated alcohol exposure from otherexposures that potentially increase the risk of preterm birth, and wecaptured uncontrolled confounding by previous reproductiveexperiences or fetal complications.Materials and methodsStudy population and data collectionThis study is a subproject of the Norwegian Mother and ChildCohort Study (MoBa) conducted by the Norwegian Institute ofPublic Health [24]. MoBa is a prospective pregnancy cohortcomprising all pregnant women in Norway attending routineultrasound examinations at gestational weeks 17e18. Recruitmentstarted in 1999 and ended in 2008, and participants were recruitedto the study through a postal invitation after they have signed upfor the routine ultrasound examination in their local hospital. Of thetotal, 40% of the invited women consented to participate, and thecohort now includes 112,768 pregnancies [25].Questionnaires used in MoBa can be found online at http://www.fhi.no/moba-en. The record of the pregnancy and deliveryin the Medical Birth Registry of Norway (MBRN) is included in theMoBa database. The information in MBRN is based on antenatalforms and data recorded at the maternity departments at deliveryand during the hospital stay [26]. The present study was based onversion 7 of the quality-assured data files made available in 2013.Written informed consent was obtained from all participatingwomen, and the study has been approved by the Regional Committeefor Ethics in Medical Research and the Data Inspectorate.VariablesWe defined preterm delivery as delivery before 37 weeks(259 days) of gestation. Predictions from ultrasound measuresdetermined the outcome variable gestational age, or if this wasmissing, from the date of the last menstrual period. Information onmaternal age at child birth was categorized as “less than 20 years,”“20e24 years,” “25e29 years,” “30e34 years,” and “35 years ormore.” The variables gestational age, maternal age, and child’s sexwere drawn from the MBRN.Questionnaire Q1, with assessment point at gestation week 15,asks about alcohol use during the last 3 months before pregnancyand during pregnancy. The woman was asked how often shedrinks alcohol with response categories “6e7 times,” “4e5 times,”“2e3 times,” “1 time per week,” “2e3 times per month,” “less thanonce per month,” or “never.” There was also a question about howmany units (for instance a glass of beer or a glass of wine) themothers usually consumed on each occasion. Table 1 gives thedistribution of these variables. If the woman reported to be anondrinker before pregnancy and did not respond to the pregnancyintake question, we assumed that she was also a nondrinkerduring pregnancy.The response categories on maternal smoking were“nonsmoker,” “occasional smoker,” and “daily smoker.” Maternaleducational attainment was categorized as “less than 12 years,”“12 years,” “13e16 years,” and “17 years or more.” The smokingvariable and the information on maternal education were takenfrom Q1 together with variables assessing prepregnancy maternalheight and weight and maternal anxiety. Prepregnancy height andbody weight were used to calculate body mass index (kg/m2) whichwas categorized as “less than 20,” “20e24,” “25e29,” and “30 ormore.” The mothers anxiety scores range from 2 to 8 and was a sumof responses “not bothered ¼ 1,” “a little bothered ¼ 2,” “quitebothered ¼ 3,” or “very bothered ¼ 4” on two questions on whether276 M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282the woman during the past two weeks have been (1) frightened oranxious or (2) nervous and in turmoil. All questionnaire variablesalso include a missing category.For the background variables (Tables 2 and 3), we includedadditional variables from Q1. First, social support was measured bythe item “Do you have anyone other than your husband/partnerthat you can ask for support in a difficult situation?” and wasdichotomized as “no” and “yes.” Second, maternal distress wasmeasured by the SCL-5, a shortened version of the HopkinsSymptom Checklist shown to correlate strongly with the SCL-25index [27]. SCL-5 has five items, with four response categoriesfrom “1 ¼ not bothered” to “4 ¼ very bothered.” A typical item was“Worrying too much about things.” We scored the index as themean of the item scores, where an average score less than 2 isconsidered within the normal range on the SCL-5. Third, wellbeingcomprises five items rated on a 7-point Likert scaleranging from “1 ¼ strongly disagree” to “7 ¼ strongly agree.” Theitems were phrased as follows: “in most ways my life is close tomy ideal,” “the conditions of my life are excellent,” “I am satisfiedwith my life,” “so far I have gotten the important things I want inlife,” and “if I could live my life over, I would change almostnothing.” Fourth, the mother was asked if she did exercise “once aweek or more” before pregnancy, and we dichotomized the responsesas “no” and “yes”. Fifth, we asked about folate intake anddichotomized the responses as “no” and “yes.” Sixth, if the womanwas previously treated for infertility, we dichotomized the responsesas “no” and “yes.” Finally, paternal alcohol use during thelast 6 months before pregnancy and after pregnancy was asked for,and the categories were: “6e7 times,” “4e5 times,” “2e3 times,”“1 time per week,” “2e3 times per month,” “less than once permonth,” or “never.” We dichotomized the responses as “Nondrinkers”and “Drinkers.”Analytic strategyAlcohol intake in pregnancy reported in Q1 was relativelyinfrequent, and the number of units consumed on each occasionwas small (see Table 1). Considering the low number of heavydrinkers in pregnancy, we chose to create a simple alcohol variable(drinker or nondrinker) for two of the time periods given in Table 1for the further analyses. In the analyses, we excluded records withmissing values for the main exposure (alcohol intake in pregnancy)but included a missing category for the potentially confoundingvariables.Multivariable logistic regression analyses were used to approximatecrude and adjusted relative risks of preterm birth accordingto alcohol intake.According to World Health Organization’s International Classificationof Diseases, Tenth Revision, the perinatal period commencesafter gestation week 22. As a result, earlier births were excluded inthis study [28]. The number of pregnancies registered in the MBRNthat had gestational length above 22 weeks and shorter than44 weeks was 107,978. Furthermore, we wanted to isolate alcoholexposure from other exposures reported to increase the risk ofTable 1Proportions of women who drank alcohol during the 3 months before pregnancy and during pregnancy, as reported in week 15 (Q1)Alcohol consumption Frequency Alcohol consumption Amount (units) per occasion*N ¼ 44,300 N ¼ 44,300 N ¼ 40,515 N ¼ 4,121Before pregnancy, n (%) Week 0e15, n (%) Before pregnancy, n (%) Week 0e15, n (%)Never 3040 (7.7) 35851 (80.9)<1 per mo 10163 (22.9) 3280 (7.4) <1 1492 (3.7) 3111 (75.5)1e3 per mo 16573 (37.4) 674 (1.5) 1e2 12637 (31.2) 624 (15.1)1 per wk 8917 (20.1) 131 (0.3) 3e4 13292 (32.8) 51 (1.2)2e3 per wk 4293 (9.7) 24 (0.1) 5e6 8444 (20.8) 34 (0.8)4e5 per wk 463 (1.0) 4 (0.0) 7e9 3206 (7.9) 9 (0.2)6e7 per wk 106 (0.2) 8 (0.0) 10þ 782 (1.9) 6 (0.1)No response 381 (0.9) 4328 (9.8) No response 662 (1.6) 286 (6.9)
Subjects who responded “never” and nonresponders to the frequency question are not included.Table 2Characteristics of women according to prepregnant intake of alcoholBackground variables Nondrinker* (n ¼ 3404) Drinker* (n ¼ 40,515) c2/F PyMotherGestational week (mean SD) 39.3 2.1 39.5 2.0 6.9 .001Age when child is born (mean SD) 27.5 4.6 28.4 4.4 69.3 <.0001 Education > 12 y (%) 60.8 71.7 248.3 <.0001Smoking during pregnancy, yes (%) 5.0 8.4 47.7 <.0001Prepregnancy BMI (mean SD) 23.8 4.6 23.7 4.3 0.8 .469Folate, yes (%) 72.4 80.2 116.4 <.0001Infertility, yes (%) 12.2 8.0 87.0 <.0001Social support, yes (%) 91.4 97.8 476.3 <.0001Exercise before pregnancy once a week (%) 75.2 83.7 124.6 <.0001Psychological distress gestation week 18, raw score from 1 to 4 (mean SD) 1.3 0.4 1.3 0.4 6.7 .001Well-being gestation week 18 raw score from 1e7 (mean SD) 5.6 1.2 5.7 1.1 4.3 <.014ChildBoys (%) 51.4 51.3 0.9 .472Birth weight, g (mean SD) 3597.4 597.3 3592.1 580.6 0.8 .378FatherDrinking before pregnancy, yes (%) 99.7 99.4 3.1 .085ANOVA ¼ analysis of variance.
Nondrinkers are women who reported being a nondrinker. Drinkers are women who responded to the frequency categories varying from “less than 1 per month” up to“6e7 times per week” and/or reported drinking “less than 1” up to “10 or more” units of alcohol on each occasion.y P values were calculated using ANOVA for continuous variables and c2 test for categorical variables.M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282 277preterm birth. Therefore, only cases defined as singleton birthswere eligible for this study (n ¼ 104,225). To capture uncontrolledconfounding by previous reproductive experiences, fetal complicationsand/or change of behavior after adverse pregnancy outcomesor opposite, we excluded multiparous women (57,973). Thefinal sample consisted of a total of 46,252 primiparous motherswith singleton births (see Fig. 1).Sensitivity analysisTo be sure that exclusion of earlier preterm births (<22 week) did not induce selection bias, we included them in a sensitivity analysis to investigate if these women were drinkers either before pregnancy (T1) or during pregnancy (T2), and if that could potentially influence our risk estimates in the primary analysis. Furthermore, to investigate the potential impact of the nonresponse of mothers on alcohol intake during pregnancy, we performed another sensitivity analysis. We repeated the primary analysis before (T1) and during pregnancy (T2) by recoding the nonresponse of mothers as first being drinkers and later as nondrinkers. By doing this, we could separately estimate the potential largest deviations: the maximum risk effect and the maximum protective effect of alcohol consumption if the missing responses were either possible drinkers or possible nondrinkers. Covariates Factors that could potentially influence maternal alcohol use and risk of preterm delivery were identified as confounders. The following covariates were included: Maternal age at delivery, years of education, pre-pregnancy body mass index, smoking, exercise before pregnancy, and paternal drinking before pregnancy. The analyses were computed with PASW software, version 22.0, (SPSS Inc., Chicago, IL). Results Characteristics of the study population Alcohol consumption before and during pregnancy was measured by a general questionnaire sent out in gestational week 15, Q1, with a response rate of 94.9%. In this version, the total sample sizes were 101,769 for Q1 and 108,327 for the MBRN, but after excluding multiparous women, plural births, and births before gestation week 22, and after gestation week 43, it was reduced to 46,252 (see flow chart concerning excluded cases for details). Nondrinkers and drinkers differed on several background variables, dependent on the two measurement times.Women drinking before pregnancy were more likely to be older, smokers, have higher education, take folic acid during pregnancy, experienced more social support, exercised more before pregnancy, and they reported less treatment due to infertility when compared with nondrinkers before pregnancy (see Table 2). Women drinking during pregnancy shared the same background variables as nondrinking women during pregnancy when it comes to age, smoking, and education. On the other hand, it was less likely that they reported taking folic acid supplements, and they experienced lower well-being, and they reported more treatment due to infertility when compared to nondrinkers during pregnancy. They also experienced more psychological distress had lower body mass index, and their partners were more likely to drink when compared to nondrinkers during pregnancy (see Table 3). Table 3 Characteristics of women according to intake of alcohol during pregnancy Background variables Nondrinker* (n ¼ 35,851) Drinker* (n ¼ 4121) c2/F Py Mother Gestational week (mean SD) 39.5 2.0 39.5 2.0 0.6 .558 Age when child is born (mean SD) 28.2 4.4 30.1 4.4 367.1 <.0001 Education > 12 y (%) 71.1 77.6 226.9 <.0001Drinking before pregnancy, yes (%)Smoking during pregnancy, yes (%) 7.5 12.0 100.7 <.0001Prepregnancy BMI (mean SD) 23.7 4.4 23.4 3.8 17.3 <.0001Infertility, yes (%) 8.5 6.6 43.5 <.0001Folate, yes (%) 80.1 76.7 25.9 <.0001Social support, yes (%) 97.4 97.9 4.1 .049Exercise before pregnancy once a week (%) 83.1 83.3 0.1 .714Psychological distress gestation week 18, raw score from 1 to 4 (mean SD) 1.2 0.4 1.3 0.4 51.7 <.0001Well-being gestation week 18 raw score from 1e7 (mean SD) 5.7 1.1 5.6 1.1 50.0 <.0001ChildBoys (%) 51.2 51.2 0.4 .263Birth weight, g (mean SD.) 3592.9 580.8 3598.1 57464 0.8 .376FatherDrinking before pregnancy, yes (%) 99.5 98.7 25.8 <.0001Drinking during pregnancy, yes (%) 95.6 99.4 96.1 <.0001
Nondrinkers are women who reported being a nondrinker. Drinkers are women who responded to the frequency categories varying from “ less than 1 per month” up to “6-7 times per week” and/or reported drinking “less than 1” up to “10 or more” units of alcohol on each occasion.y P values were calculated using ANOVA for continuous variables and c2 test for categorical variables.Fig. 1. Flow diagram displaying exclusion criteria for the final study population.*GW ¼ Gestational weeks. *Variables from MBRN. 278 M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282 Effect of alcohol and risk of preterm birth In this dataset, the prevalence of preterm birthwas 5.9% (2729 of 46252). The median length of gestation was 40 weeks for both drinkers during pregnancy and nondrinkers, whereas the mean gestational lengthwas 39.47weeks (SD ¼ 2.03) for both groups. The mean pregnancy length for prepregnancy drinkers was 39.48 (SD ¼ 2.02), whereas it was 39.34 (SD ¼ 2.12) for nondrinkers (F ¼ 13.4, 1 degrees of freedom, P < .001). The mean pregnancy length for drinkers during pregnancy was 39.50 (SD ¼ 2.02) and 39.47 (SD ¼ 2.03) for nondrinkers (F ¼ 1.2, 1 degrees of freedom, P ¼ .281). Overall, alcohol consumption was not associated with preterm birth risk. Table 4 lists a statistically significant tendency toward lower risk of preterm birth of prepregnancy drinking (adjusted odds ratio [aOR] ¼ 0.81, 95% CI ¼ 0.70e0.95). When we analyzed the dose-response relationship, we found that the risk decreased when the average frequency in consumption increased. A similar association was not found among women who reported drinking during early pregnancy aOR ¼ 1.03 (95% CI ¼ 0.90e1.19). Table 5 shows the same tendency: when women who reported being drinking either before pregnancy or during the first 15 gestation weeks were analyzed together with subjects who reported drinking in both periods, the risk of preterm birth was significantly reduced for women who reported drinking before pregnancy. But not after pregnancy, with nondrinkers as the reference (aOR ¼ 0.80, 95% CI ¼ 0.68e0.94). Potential effect of very early preterm birth and nonresponse on alcohol intake In this data set (after exclusion of multiple births and parity), 93 children were born before gestation week 22 and eligible to be included in the sensitivity analysis. These extra cases increased the total number of preterm births from 2474 to 2567 at T1 and from 2346 to 2439 at T2. When we explored the potential impact of nonresponse mothers, the total number of drinkers increased from 40,515 to 40,896 and from 3404 to 3785 for nondrinkers at T1. At T2, the total number of drinkers increased from 4121 to 8449 and from 35,851 to 40,179 for nondrinkers. As Table 6 shows, our findings in the main analysis remained the same after exploring the potential impact of very early preterm birth and the nonresponse of mothers. When we included very early preterm births in the analysis, the estimates were in line with our previous results both before pregnancy (aOR ¼ 0.84, 95% CI ¼ 0.73e0.96) and during pregnancy (aOR ¼ 1.03, 95% CI ¼ 0.90e1.18). Whenweincluded mothers with nonresponse in the analysis and recoded them as being drinkers, the odds for preterm birth were unchanged with no risk for prepregnancy drinking (aOR ¼ 0.82, 95% Table 4 Number and prevalence (percent) of preterm births (<37 weeks), and crude odds ratios and aORs according to self-reported alcohol intake before pregnancy (T1) and the first 15e17 weeks of pregnancy (T2) for 46,252 pregnancies in the Norwegian Mother and Child Cohort Study First-time mothers and preterm birth (T1 alcohol before pregnancy) Exposure N of births N of cases Percent cOR aORNondrinker T1y 3404 236 6.93 1.0 1.0Drinker T1y 40,515 2338 5.77 0.82 (0.72e0.94)z 0.81 (0.70e0.95)z<1 per mo 10,163 629 6.19 0.89 (0.76e1.03) 0.85 (0.71e1.01)1e3 per mo 16,573 989 5.97 0.85 (0.74e0.99)z 0.85 (0.72e1.00)x1 per wk 8917 486 5.45 0.77 (0.66e0.91)z 0.76 (0.64e0.92)z2e3 per wk 4293 210 4.89 0.69 (0.57e0.84)z 0.66 (0.53e0.83)z4e5 per wk 463 19 4.10 0.57 (0.36e0.93)z 0.58 (0.34e0.99)z6e7 per wk 106 5 4.71 0.67 (0.27e1.65) 0.61 (0.22e1.71)No response 381 24 6.23 0.90 (0.52e1.39) 1.45 (0.80e2.63)First-time mothers and preterm birth (T2 alcohol during first 15 wk)Exposure N of births N of cases Percent cOR aORkNondrinker T2y 35,851 2104 5.87 1.0 1.0Drinker T2y 4121 242 5.87 1.00 (0.87e1.15) 1.03 (0.90e1.19)<1 per mo 3280 180 5.49 0.93 (0.80e1.09) 0.97 (0.82e1.13)1e3 per mo 674 55 8.16 1.43 (1.08e1.89)z 1.51 (1.14e2.00)z1 per wk 167 7 3.82 0.70 (0.33e1.50) 0.71 (0.33e1.52)No response 4328 252 6.18 0.99 (0.87e1.14) 0.99 (0.86e1.14)
Adjusted for maternal age, educational attainment, smoking before pregnancy, prepregnancy BMI, exercise before pregnancy and paternal drinking before pregnancy.y Nondrinkers are women who reported being a nondrinker. Drinkers are women who responded to the frequency categories varying from “ less than 1 per month” up to“6e7 times per week” and/or reported drinking “less than 1” up to “10 or more” units of alcohol on each occasion.z P < .05.x Borderline statistically significant.k Adjusted for drinking before pregnancy, maternal age, educational attainment, smoking before pregnancy, pre-pregnancy BMI and paternal drinking before pregnancy.Table 5Prevalence of preterm birth and crude (cOR) and adjusted (aOR) odds ratios (with 95% confidence interval (CI)) according to alcohol intake before pregnancy and during first15 weeks of gestation in 46, 252 pregnancies from the Norwegian Mother and Child Cohort StudyFirst-time mothers and preterm birth (T1 and T2)Drinking before pregnancy* Drinking 0e15 wk* No of births No of cases Prevalence cOR aORyNo No 3160 235 4.18 1.0 1.0Yes No 30,523 1862 4.14 0.82 (0.71e0.94)z 0.80 (0.68e0.94)zYes Yes 3857 239 3.63 0.83 (0.69e1.00)x 0.83 (0.66e1.03)
Women who report being drinking either before pregnancy or during the first 15 gestation weeks were analyzed together with subjects who report drinking in bothperiods.y Adjusted for maternal age, educational attainment, smoking before pregnancy, prepregnancy BMI, and paternal drinking before pregnancy.z P < .05. x Borderline statistically significant. M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282 279 CI ¼ 0.70e0.97) and drinking during pregnancy (aOR ¼ 0.95, 95% CI ¼ 0.85e1.07). The estimates remained stable showing no risk of preterm birth associated with alcohol consumption when we recoded nonresponse as nondrinkers at T1 (aOR ¼ 0.80, 95% CI ¼ 0.69e0.94) and T2 (aOR ¼ 1.01, 95% CI ¼ 0.86e1.17). Discussion The message from this study is somewhat complex. Our finding that alcohol consumption during the last 15 weeks before pregnancy was associated with reduced risk of preterm birth, whereas drinking during pregnancy was not is intuitively difficult to make sense of. Notably, we did not observe an overall effect on preterm birth among alcohol drinkers during pregnancy. Instead, the doseresponse analyses showed tendencies toward adverse effects when drinking 1e3 times per month during the first trimester. This finding is not in accordance with the two Danish prospective studies of alcohol exposure during pregnancy and preterm birth that report a J-shaped relationship between the dose of alcohol and the risk of preterm birth, indicating that moderate drinking during pregnancy is protective [11,12]. An important contrast to these aforementioned studies is that the participants of MoBa report very low alcohol consumption. We had no possibility of studying the effects of alcohol intake above four drinks per week. Another essential difference between this study and the two Danish studies is that the risk reduction was observed for drinking before pregnancy. Our results suggest a protective dose-response effect of prepregnancy alcohol consumption. Women who consumed alcohol 4e5 times per week had a lower risk (aOR ¼ 0.58) than women drinking less than once per month (aOR ¼ 0.85). This reduced risk with prepregnancy drinking has previously been reported in a review by Patra et al. (2011) [4]. On the other hand, their review showed a dose-response relationship between preterm birth and prenatal drinking, with no effect at an average of 1.5 drinks per day and monotonically increased risk with higher consumptions. Cloner and Rezkalla (2007) [29] pose the question “To drink or not to drink” in their review of the benefits of low to moderate consumption alcohol. One might suggest that a more relevant question is about the labeling “To be a drinker or a nondrinker?” rather than the alcohol consumption by itself. We assume that a possible and more intuitive explanation can be that differences in socioeconomic status, health-related behavior, and physical and psychological well-being between drinkers and nondrinkers influence the risk estimates. Interestingly, alcohol consumption among women in northern Europe and the United States has increased during the last decades as alcohol has become more integrated into everyday life. The alcohol consumption in Norway has increased by 40 percent over the last two decades, with the highest growth among welleducated and high-income women [30]. In most western cultures, the use of alcohol is the norm, and abstinence is associated with social deviance [31,32]. A relevant Norwegian population-based study shows that people with low alcohol consumption are at higher risk of depression and anxiety compared with moderate drinkers, and the abstainers have the highest risk of experiencing these negative mental health consequences [32]. This tendency has previously been described as the “healthy drinker effect.” Healthy women tend to drink more than women with chronic diseases [15,18]. However, perhaps a broader term such as the “higher well-being effect” is more appropriate, as intake of alcohol appears to be associated with healthful lifestyle, higher education, and better social network. In this study, we identified several significant differences in background variables between drinkers and nondrinkers. Drinkers overall appear to have higher education, be more social and have a higher score onwell-being. Not surprisingly, the differences aremost Table 6 Sensitivity analyses with number and prevalence (percent) of preterm births (<37 weeks), and crude (cOR) and adjusted (aOR) odds ratios according to self-reported alcohol intake before pregnancy (T1) and the first 15e17 weeks of pregnancy (T2) including very preterm births (>22) and the nonresponse of mothers on alcohol intake in the mainanalysisFirst-time mothers and preterm birth (T1 alcohol before pregnancy)Exposure N of births N of cases Percent cOR aOR*Including very early preterm birth (<22, n ¼ 88)Nondrinker T1y 3411 243 7.12 1.0 1.0Drinker T1y 40,596 2419 5.96 0.83 (0.72e0.95)z 0.84 (0.73e0.96)zIncluding nonresponse as drinkers (n ¼ 381)Nondrinker T1 3404 236 6.93 1.0 1.0Drinker T1 40,896 2362 5.78 0.82 (0.72e0.95)z 0.82 (0.70e0.97)zIncluding nonresponse as nondrinkers (n ¼ 381)Nondrinker T1 3785 260 6.87 1.0 1.0Drinker T1 40,515 2338 5.77 0.83 (0.73e0.95)z 0.80 (0.69e0.94)zFirst-time mothers and preterm birth (T2 alcohol during first 15 wk)Exposure N of births N of cases Percent cOR aORxIncluding very early preterm birth (<22, n ¼ 81)Nondrinker T2 35,919 2172 6.05 1.0 1.0Drinker T2 4134 255 6.17 1.02 (0.89e1.17) 1.03 (0.90e1.18)Including nonresponse as drinkers (n ¼ 4328)Nondrinker T1 35,851 2104 5.12 1.0 1.0Drinker T1 8449 494 5.85 0.97 (0.90e1.10) 0.95 (0.85e1.07)zIncluding nonresponse as nondrinker (n ¼ 4328)Nondrinker T1 40,179 2356 5.86 1.0 1.0Drinker T1 4121 242 5.87 1.00 (0.87e1.15) 1.01 (0.86e1.17)z
Adjusted for maternal age, educational attainment, smoking before pregnancy, prepregnancy BMI, exercise before pregnancy and paternal drinking before pregnancy.y Nondrinkers are women who reported being a nondrinker. Drinkers are women who responded to the frequency categories varying from “ less than 1 per month” up to“6e7 times per week” and/or reported drinking “less than 1” up to “10 or more” units of alcohol on each occasion.z P < .05.x Adjusted for drinking before pregnancy, maternal age, educational attainment, smoking before pregnancy, prepregnancy BMI and paternal drinking before pregnancy.280 M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282pronounced before pregnancy, since during pregnancy total alcoholabstinence is highly recommended, and therefore, the socioeconomicdifference between drinkers and nondrinkers decreases atthat measurement time. Consequently, it is very likely that studieslike our study, measuring the association between the mothers’ selfreportedalcohol intake and preterm risk, also indirectly measuresthe impact of maternal social resources and well-being by assessingseveral factors that accompany both alcohol and the risk of pretermbirth. The fact that adverse social circumstances are associated withthe risk of preterm birth underscores the complexity of factors andmechanisms involved in understanding the association betweenalcohol consumption and potential adverse birth effects [9,33]. Inthis sense, if alcohol consumption was the only significant predictorand explanation for our results, the exposure should display thesame risk estimates during early pregnancy, and not only be associatedwith reduced preterm risk when measured before pregnancy.If there is a biological explanation for this association, themechanisms are uncertain. One possible explanation of the results issimilar causal pathways as for the association between alcohol andcardiovascular disease. Studies have found protective effects ofalcohol on the vascular endothelium, resulting in increased angiogenesis,and reduced risk of inflammation and infections [33].However, as already mentioned, this protective effect does notpertain in our studywhenwomen report drinking during pregnancy.Strengths of this study include a large population sample of firsttimemothers with only singleton births, with relevant data on anumber of covariates and background variables that permittedadjustment for important confounders. Alcohol exposure wasassessed in the first trimester prospective to the birth outcome,avoiding potential systematic errors due to under-reporting andrecalls bias. Our relatively large cohort of mothers should enableidentification of variations in maternal attributes, and a study ofrecruitment bias in MoBa by Nilsen et al. (2009) [34] concludes thatexposure-outcome associations are not biased.However, this cohort study suffers from several limitations thatmight have affected the validity of our findings. As with otherquestionnaire studies, some caution is required when interpretingself-reported measures. Mothers’ responses may be affected bysocial desirability and other response biases. Women who believethat they should not drink any alcohol during pregnancy mayunder-report alcohol intake, and cause a potential source of error bymisclassification toward inclusion of drinkers in the nondrinkercategory. Such misclassification may lead to a bias toward the nullvalue in the estimate of the effects of alcohol consumption duringpregnancy on preterm birth.Moreover, drinking before pregnancy were assessed retrospectivelyand is potentially subject to recall bias and under-reportingespecially if mothers discover any complications during earlypregnancy. For this purpose, we performed a sensitivity analysis onthe potential impact of the nonresponding mothers and motherswith births before gestation week 22, to see if these groups couldinduce selection bias. However, the effect estimates in the cohortanalyses remained unchanged. If under-reporting were present inthe self-reported alcohol consumption, the effect estimates shouldhave been underestimated.A relevant substudy of the MoBa cohort explored the risk ofunderestimates with variables linked to social desirability. Theyfound that self-reported smoking status during pregnancy had asensitivity of 82% and a specificity of 99% against plasma cotinineconcentrations, signifying that self-reported smoking is a validmeasure in the MoBa cohort [35]. This could suggest that reportingbias is not the explanation for our results, especially before pregnancy,as there is no established stigma attached to prepregnancydrinking in Norway and most of mothers in the cohort (91%) reportto drink alcohol.Another possible source of error is that the timing of conceptionand alcohol consumption may overlap. Other studies have shownthat when women are asked about their alcohol consumptionbefore pregnancy, they might report consumption before pregnancyrecognition and therefore include early pregnancy [36,37].An important limitation is that even if potentially confoundingvariables are included in the data set, residual confounding cannotbe ruled out: The list of potential confounders is extensive, and it isimpossible to control for all. Finally, two factors in our study raise animportant question of the representativeness of the sample. First,the missing rate is high, and the women who did not participatemight have a different exposure-outcome relation than those whodid participate. Second, given that we only included primiparouswomen with singleton births, our findings are limited to representwomen at low risk. Hence, our conclusions might not provide anyguidance to other pregnant women. Consequently, we suggest thatfurther studies should explore the effects of alcohol consumptionamong different groups of women.ConclusionsWe cannot conclude from our study that alcohol intake is ofbenefit to the fetus. More research is needed to understand thecausal pathways. The health of the fetus may be affected in manycomplex ways by alcohol, and any beneficial effect on reduced riskof preterm birth may as well be outweighed by other risks. Thisquestion will be investigated further in MoBa by looking at neurodevelopmentaloutcomes. Furthermore, the protective effect ofprepregnancy alcohol consumption, also seen in other studies, canreflect a biological mechanism where geneeenvironment interactionshave a part to play and that may lead to better preventionof preterm birth after more detailed clinical and experimentalstudies have explored the possible underlying mechanisms.As this effect was not present during pregnancy, we furthersuggest that any direct positive effects of prepregnancy drinkingcan be explained by differences in socioeconomic backgroundvariables that we have not been able to control for. More importantly,our data suggest that women drinking during pregnancy arenot at lower risk of having a preterm birth. Instead, there might be aslight inverse risk that warrants the existing recommendationsagainst any maternal alcohol intake during pregnancy.AcknowledgmentsAll authors have contributed to the conception and design of thestudy, revising the article critically for important intellectual contentand given the final approval of this version to be submitted.The study is supported by The Norwegian Ministry of Health: NIH/NIEHS (grant no. N01-ESe85433), NIH/NINDS (grant no. 1 UO1 NS047537) and the Norwegian Research Council/FUGE (grant no.151918/S10). Maria T.G. Dale has received funding from an unrestrictedgrant from Oak Foundation, Geneva, Switzerland. The studysponsors were not involved in the study design; in the collection,analysis, or interpretation of data; in the writing of the report; or inthe decision to submit the article for publication.References[1] Report No. IS-1284 Alcohol and pregnancy, report from an expert group. Oslo,Norway: Norwegian Health Directorate; 2005 https://helsedirektoratet.no/Lists/Publikasjoner/Attachments/24/Alkohol-og-graviditet-hva-er-farlig-forfosteret-hvordan-forebygge-og-behandle-IS-1284.pdf [accessed 09.04.2015].[2] International guidelines on drinking during pregnancy. Washington, D.C.,USA: Washington, D.C., USA: International Center for Alcohol Policies; 2009http://www.icap.org/Table/InternationalGuidelinesOnDrinkingAndPregnancy[accessed 09.04.2015].M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282 281[3] Henderson J, Gray R, Brocklehurst P. Systematic review of effects of lowmoderateprenatal alcohol exposure on pregnancy outcome. BJOG2007;114(3):243e52.[4] Patra J, Bakker R, Irving H, Jaddoe VW, Malini S, Rehm J. Doseeresponserelationship between alcohol consumption before and during pregnancy andthe risks of low birthweight, preterm birth and small for gestational age(SGA)da systematic review and meta-analyses. BJOG 2011;118(12):1411e21.[5] Jones K, Smith D. Recognition of the fetal alcohol syndrome in early infancy.Lancet 1973;302(7836):999e1001.[6] Little RE, Wendt JK. The effects of maternal drinking in the reproductiveperiod: An epidemiologic review. J Subst Abuse 1991;3(2):187e204.[7] Ouellette EM, Rosett HL, Rosman NP, Weiner L. Adverse effects on offspring ofmaternal alcohol abuse during pregnancy. N Engl J Med 1977;297(10):528e30.[8] Blencowe H, Cousens S, Oestergaard MZ, Chou D, Moller A-B, Narwal R,et al. National, regional, and worldwide estimates of preterm birth rates inthe year 2010 with time trends since 1990 for selected countries: asystematic analysis and implications. Lancet 2012;379(9832):2162e72.[9] Hoffman HJ, Bakketeig LS. Risk factors associated with the occurrence ofpreterm birth. Clin Obstet Gynecol 1984;27(3):539e52.[10] The prevention of perinatal mortality and morbidity. Report of a WHO ExpertCommittee. World Health Organ Tech Rep Ser 1970;457:1e60.[11] Medical birth registry of Norway annual report. Oslo, Norway: NorwegianInstitute of Public Health; 2014 http://www.fhi.no/dokumenter/ba886ffc80.pdf [accessed 09.04.2015].[12] Saigal S, Doyle LW. An overview of mortality and sequelae of preterm birthfrom infancy to adulthood. Lancet 2008;371(9608):261e9.[13] Goldenberg RL, Culhane JF, Iams JD, Romero R. Epidemiology and causes ofpreterm birth. Lancet 2008;371(9606):75e84.[14] Albertsen K, Andersen A-MN, Olsen J, Grønbæk M. Alcohol consumptionduring pregnancy and the risk of preterm delivery. Am J Epidemiol2004;159(2):155e61.[15] Kesmodel U, Olsen SF, Secher NJ. Does alcohol increase the risk of pretermdelivery? Epidemiology 2000;11(5):512e8.[16] Wright SP, Mitchell EA, Thompson JM, Clements MS, Ford RP, Stewart AW.Risk factors for preterm birth: a New Zealand study. N Z Med J1998;111(1058):14e6.[17] Meis PJ, Goldenberg RL, Mercer BM, Iams JD, Moawad AH, Miodovnik M, et al.The preterm prediction study: risk factors for indicated preterm births. Am JObstet Gynecol 1998;178(3):562e7.[18] Lundsberg LS, Illuzzi JL, Belanger K, Triche EW, Bracken MB. Low-to-moderateprenatal alcohol consumption and the risk of selected birth outcomes:a prospective cohort study. Ann Epidemiol 2015;25(1):46e54.[19] Jaddoe VWV, Bakker R, Hofman A, Mackenbach JP, Moll HA, Steegers EAP, et al.Moderate alcohol consumption during pregnancy and the risk of low birthweight and preterm birth. The Generation R Study. Ann Epidemiol2007;17(10):834e40.[20] Lundsberg LS, Bracken MB, Saftlas AF. Low-to-moderate gestational alcoholuse and intrauterine growth retardation, low birthweight, and preterm delivery.Ann Epidemiol 1997;7(7):498e508.[21] Andersen A-MN, Andersen PK, Olsen J, Grønbæk M, Strandberg-Larsen K.Moderate alcohol intake during pregnancy and risk of fetal death. Int J Epidemiol2012;41(2):405e13.[22] Stene-Larsen K, Torgersen L, Strandberg-Larsen K, Normann PT,Vollrath ME. Impact of maternal negative affectivity on light alcohol useand binge drinking during pregnancy. Acta Obstet Gynecol Scand2013;92(12):1388e94.[23] Caetano R, Ramisetty-Mikler S, Floyd LR, McGrath C. The epidemiology ofdrinking among women of child-bearing age. Alcohol Clin Exp Res2006;30(6):1023e30.[24] Magnus P, Irgens LM, Haug K, Nystad W, Skjærven R, Stoltenberg C, et al.Cohort profile: The Norwegian Mother and Child Cohort Study (MoBa). Int JEpidemiol 2006;35(5):1146e50.[25] Schreuder P, Alsaker ER. The Norwegian Mother and Child Cohort Study(MoBa) e MoBa recruitment and logistics. Nor Epidemiol 2014;24(1-2):23e7.[26] Irgens LM. The Medical Birth Registry of Norway. Epidemiological researchand surveillance throughout 30 years. Acta Obstet Gynecol Scand2000;79(6):435e9.[27] Strand BH, Dalgard OS, Tambs K, Rognerud M. Measuring the mentalhealth status of the Norwegian population: a comparison of the instrumentsSCL-25, SCL-10, SCL-5 and MHI-5 (SF-36). Nord J Psychiatry2003;57(2):113e8.[28] Langhoff-Roos J, Kesmodel U, Jacobsson B, Rasmussen S, Vogel I. Spontaneouspreterm delivery in primiparous women at low risk in Denmark: populationbased study. BMJ 2006;332(7547):937e9.[29] Kloner RA, Rezkalla SH. To drink or not to drink? That is the question. Circulation2007;116(11):1306e17.[30] Annual report. Oslo, Norway: Norwegian Institute of Public Health; 2014http://www.fhi.no/dokumenter/4313e1cf39.pdf [accessed 09.04.2015].[31] Peele S, Brodsky A. Exploring psychological benefits associated with moderatealcohol use: a necessary corrective to assessments of drinking outcomes?Drug Alcohol Depend 2000;60(3):221e47.[32] Skogen JC, Harvey SB, Henderson M, Stordal E, Mykletun A. Anxiety anddepression among abstainers and low-level alcohol consumers. The Nord-Trondelag Health Study. Addiction 2009;104(9):1519e29.[33] Peacock JL, Bland JM, Anderson HR. Preterm delivery: effects of socioeconomicfactors, psychological stress, smoking, alcohol, and caffeine. BMJ1995;311(7004):531e5.[34] Nilsen RM, Vollset SE, Gjessing HK, Skjærven R, Melve KK, Schreuder P, et al.Self-selection and bias in a large prospective pregnancy cohort in Norway.Paediatr Perinat Epidemiol 2009;23(6):597e608.[35] Kvalvik LG, Nilsen RM, Skjaerven R, Vollset SE, Midttun O, Ueland PM, et al.Self-reported smoking status and plasma cotinine concentrations amongpregnant women in the Norwegian Mother and Child Cohort Study. PediatrRes 2012;72(1):101e7.[36] Day NL, Cottereau CM, Ricardson GA. The epidemiology of alcohol, marijuana,and cocaine use among women of childbearing age and pregnant women. ClinObstet Gynecol 1993;36(2):232e45.[37] Whitehead N, Lipscomb L. Patterns of alcohol use before and during pregnancyand the risk of small-for-gestational-age birth. Am J Epidemiol2003;158(7):654e62.282 M.T.G. Dale et al. / Annals of Epidemiology 26 (2016) 275e282

Prompt: Identify the research methods used in the study, specifically addressing the following in 2-3 pages in legnth:

· Was this an observational or experimental study?

· Describe the methods used to collect the data and explain why the methods are appropriate based on the research question.

· Was the data collected quantitative or qualitative? Support your identification with examples from the study.

· Describe the analysis method used to analyze the data, identifying the software that was used.

· Discuss the potential weaknesses of the data collection and data analysis methods used. Support your discussion with specific details.

· Describe the key demographics of the population sampled and identify the inclusion and exclusion criteria for participants.

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