Results
Study Population
Questionnaires were handed out by health care workers nation-wide in 222 Well-Baby Clinics. We obtained 2,759 questionnaires of the 5,410 questionnaires handed out in 2007 and 1,448 of the 3,620 in 2010, response rates were respectively 51 % and 40 %. The questions on alcohol use were answered by 2,715 (2007) resp. 1,410 women (2010). Our sample was representative for women who gave birth these years with respect to maternal age at child birth, gestational age, and number of children and place of birth (home or hospital), but not for level of education. In order to obtain representative samples of mothers living in the Netherlands, data were weighted. Weighting factors were set with reference to data of the Dutch population provided by Statistics Netherlands. Mean birth weight ± SD in our sample was 3,477 ± 574 g, mean duration of pregnancy 39.5 weeks, 5 % of infants were born preterm (<<1 % extremely, <1 % very and 4 % moderately preterm), 6 % of infants were born SGA. Mean age of the mothers was 30.6 years (57.9 %; range 15 till 46 years), most are of first parity with a high education level. The questionnaire was filled in when their infants were on average three months old.
Table 1 shows the prevalence rates of alcohol consumption around pregnancy. Sixty-nine percent of women consumed alcohol within 6 month before pregnancy (data from 2010 only). During pregnancy the prevalence of alcohol consumption was 22 % in 2007, and 19 % in 2010 (adj. OR 0.7; 95 % CI 0.6–0.8), (p = 0.018). During the first three months of pregnancy, the prevalence was 17 % in 2007, and 14 % 2010 (p = 0.026). The first month after birth, percentages were 62 % in 2007, and 48 % in 2010 (p < 0.001).
The median frequency of alcohol consumption during pregnancy in both surveys (2007 and 2010) was <1 occasion per month (2007: 11 % vs 2010: 9 %). Roughly 4 % drank on 1–3 and only 1 % on ≥4 occasions per month, while three women indicated to consume alcohol daily. Per occasion, most pregnant women consumed <1 glass alcohol or 1–3 glasses, a 0.9 % minority consumed >3 glasses per occasion. The percentage of women who reported binge drinking (>6 glasses alcohol per occasion) during pregnancy was 0.4 % of all pregnant women. In this respect, comparison between 2007 and 2010 showed that the amount of alcohol consumed per occasion had significantly increased (p < 0,001). The group of women who consumed 1–3 or >3 glasses of alcohol per occasion had doubled between 2007 and 2010, while binge drinking had not increased.
Alcohol consumption of pregnant women stratified by level of formal education (low, medium, high) and age (<30, ≥30 years) is shown in Table 2. The percentage of women that consumed alcohol during pregnancy increased with the level of education (p < 0.001). In the highest educated group 56 % consumed alcohol versus 8 % in the lowest educated group (adj. OR 2.39; 95 % CI 1.80–3.18). A similar association was found for maternal age (p = 0.003); the prevalence of alcohol consumption is higher among women aged 30 years or older than among younger women (adj. OR 1.47; 95 % CI 1.25–1.74). Smokers, as compared to non-smokers, had a higher risk also to drink alcohol during pregnancy (adj. OR 2.05; 95 % CI 1.55–2.72).
We found no significant association between alcohol consumption during pregnancy as explanatory variable and birth weight, gestational age or weight-for-gestational age. Mothers of second-born infants less often consumed alcohol during pregnancy as compared to mothers who had their first child (adj. OR 0.79; 95 % CI 0.67–0.93).