Women who consumed low levels of alcohol before conception and during pregnancy were more likely to have adverse birth outcomes such as low birth centile, low birth weight, and premature birth than nondrinkers, a British study reported.
In the first trimester, consuming less than 2 units of alcohol per week was associated with an adjusted -5.8 (95% CI -10.8 to -0.7) decrease in customized birth centile compared with nondrinkers (Ptrend = 0.002), wrote Camilla Nykjaer, MSc, of the University of Leeds and her co-authors in the Journal of Epidemiology and Community Health. (A unit of alcohol was defined as 10 mL by volume or 8 grams by weight.)
Ingesting more than 2 units a week was associated with an adjusted -8.2 (95% CI -12.6 to -3.7) decrease in customized birth centile compared with nondrinkers (Ptrend = 0.002).
Consuming more than two units of alcohol per week in the month before conception was associated with a -7.7 (95% CI -12.8 to -2.6) decrease in customized birth centile (adjusted Ptrend=0.009) compared with nondrinkers, they wrote. (Customized birth centiles took into account maternal height, weight, ethnicity, parity, neonatal birth weight, and sex.
The adjusted odds ratios for small gestational age in the first trimester were 1.7 for those who drank less than 2 units a week (95% CI 0.9 to 3.1) and 2.0 for those who consumed more than two units a week compared with nondrinkers (95% CI 1.2 to 3.4, Ptrend= 0.03).
The low birth centile and small gestational age associations were "attenuated" in the second and third trimesters, the authors reported.
The adjusted odds ratios for giving birth prematurely were 4.6 for women who drank less than two units of alcohol a week (95% CI 1.4 to 14.7) and 3.5 for those who drank more than two units a week (95% CI 1.1 to 11.2), compared with nondrinkers, they added.
This association was "nonsignificant" in those who drank in the 4 weeks before conception and during the second and third trimester, the authors said.
The study included 1,264 pregnant women between 18 and 45 enrolled in the Caffeine and Reproductive Health (CARE) Study. Their alcohol consumption was gauged based on questionnaires administered 4 weeks before conception and during each trimester.
The questionnaires asked how often participants drank alcohol and what forms of alcohol they drank. Frequency and quantity of alcohol consumption was converted to times per week, which was then multiplied by the units of alcohol in each of the alcoholic beverages listed on the questionnaire to obtain weekly consumption in units.
Information on birth outcomes was obtained from hospital maternity records.
The researchers adjusted for caffeine and energy intake, as well as salivary cotinine levels.
Alcohol crosses the placenta "and results in nearly equal concentrations in the mother and fetus," the authors noted. But while the evidence regarding the damaging effects of heavy drinking in pregnancy has been well-established, results of studies of the impact of low levels of alcohol consumption on birth outcomes have been varied.
The lack of consensus on the impact of low levels of consumption has resulted in varying national policies with regard to alcohol consumption during pregnancy, the authors said.
In the U.S., the Surgeon General has advised women against drinking alcohol during pregnancy. In the U.K., where the study took place, the Department of Health recommends that pregnant women and those trying to conceive avoid alcohol or limit consumption to one to two units of alcohol once or twice a week.
The most recent U.K. Infant Feeding Survey (IFS), which included data from more than 15,000 women, indicated that 40% drank alcohol during their pregnancies, but that only 3% drank more than two units per week.
In this study, the proportion of women who reported drinking before and during pregnancy was considerably higher, the authors said. "Nearly two-thirds of women before pregnancy and over half in the first trimester reported alcohol intakes above the Department of Health (U.K.) guidelines of ≤2 units/week," they wrote.
The IFS data was collected postpartum and might have been subject to under-reporting, the authors cautioned. The characteristics of drinking mothers in this study and the IFS survey were generally consistent, however -- women tended to be 35 or older, from managerial and professional occupations, and from a white ethnic background.
One potential weakness of the study was that it was designed to assess caffeine intake, not alcohol consumption. "However, the questionnaire was validated with reference to caffeine intake and is comparable to other methods used in the assessment of alcohol," the authors said.
The study also suffered from low sample size in the third trimester.
"Our results highlight the need for endorsing the abstinence-only message, and further illuminate how timing of exposure is important in the association of alcohol with birth outcomes, with the first trimester being the most vulnerable period," the authors wrote.
Future studies should also take into account timing of exposure, the authors said. Maternal alcohol consumption usually decreases throughout pregnancy, they said. "Therefore, averaging exposure measured at one time point in pregnancy to reflect exposure across the whole of pregnancy may obscure any true associations."
The CARE study was supported by a grant from the Food Standard Agency. Camilla Nykjaer's PhD Studentship is funded by the Medical Research Council and the Rank Prize Foundation.
The authors disclosed no relevant relationships with industry.
- Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania
Journal of Epidemiology and Community HealthSource Reference: Nykjaer C, et al "Maternal alcohol intake prior to and during pregnancy and risk of adverse birth outcomes: evidence from a British cohort" J Epidemiol Community Health; 2014; DOI: 10.1136/jech-2013-202934.