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Effect of Maternal Alcohol Consumption on Fetal Growth and Preterm Birth
Abstract & Commentary
By John C. Hobbins, MD. Dr. Hobbins is Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver; he reports no financial relationship to this field of study. This article originally appeared in the April issue of OB/GYN Clinical Alert; for that publication it was reviewed by Catherine LeClair, MD. Dr. LeClair reports no financial relationship to this field of study.
Synposis: Low levels of alcohol consumption during pregnancy is not associated with small-for-gestational age or preterm birth. Conversely, higher levels of alcohol intake during pregnancy is associated with an increased risk of preterm birth, even after ceasing alcohol intake before the second trimester.
Source: O'Leary CM, et al. The effect of maternal alcohol consumption on fetal growth and preterm birth. BJOG 2009; 116:390-400.
It has been very difficult to study the effects of alcohol consumption on the fetus and on pregnancy, in general, because one depends so heavily on a patient's candor regarding true alcohol consumption and the effect of confounding variables, such as smoking. In Australia, drinking some alcoholic beverages in pregnancy is quite common, and, as opposed to the United States where drinking even the smallest amount of alcohol is strongly discouraged, the Australian National Health and Medical Research Council in 2001 recommended that "if women choose to drink during pregnancy, they should have less than seven standard drinks per week and, on any one day, no more than two standard drinks."1
With this backdrop in mind, a group of Australian investigators studied a random sample of 4,719 women who delivered in Western Australia between 1995 and 1997. The sample represented 10% of births in the region, and information regarding alcohol consumption was obtained via questionnaires sent out after birth. The authors were interested in two outcomes: the number of preterm births defined as those delivering before 37 weeks and the incidence of small-for-gestational age (SGA) births. To quantify the latter outcome, an optimal birth weight was calculated using the sex of the infant, maternal height, parity, and gestational age. Then the actual birth weight was matched up to this and, if it was below the 10th percentile, the infant was considered to be SGA.
As to alcohol consumption, the authors coded "low" as less than three drinks per week, "moderate" as 2-5 drinks per week, "heavy" as more than seven drinks per week, and "binging" as more than two drinks at a time (although this was difficult to sort out from the paper).
In this random sampling, about 50% of the study participants did consume some alcohol during pregnancy. Those women in the low level category who continued to drink showed little change in their drinking patterns. However, in all the other categories, there was a general decrease in the average consumption of alcohol. Interestingly, the percentage of heavy and binge drinkers decreased by two-thirds during pregnancy.
The punch line is that in each category there was no statistically significant difference in the rate of preterm birth in any category (compared with abstainers). However, if data from the low level group, representing the largest study group, were excluded from the analysis, then there was a 78% percent increase in preterm births (over abstainers). The incidence of SGA was higher among heavy and binge drinkers (13%) compared with the rate reported in the overall population (8.9%). However, since the heavy and binge group was heavily spiked with smokers, there was no difference in the incidence of SGA when the authors accounted for this confounding variable. The strangest result was that heavy drinkers who stopped drinking before the second trimester had the highest rate of preterm birth.
In a matter of 40 years, the pendulum has swung from clinicians infusing huge amounts of IV alcohol to stop preterm labor to, now, telling patients that any amount of alcohol consumed by a mother may be dangerous to the health of her fetus. In Europe, and now I realize in Australia, there is a more relaxed approach to alcohol and pregnancy. The above study does not address the effects of alcohol in small doses on the fetal brain, but it does address two issues its affect on fetal growth and preterm birth. The bottom line is that, with one exception, there is no major effect on these two outcomes, if the confounding variable of maternal smoking is taken into account.
The surprise finding in this study was that when heavy or binge drinkers stopped drinking after the first trimester, they had the highest rate of preterm birth (13%). The authors postulate that sudden abstinence "may trigger an inflammatory or other metabolic response resulting in an elevation of cytokines" responsible for preterm labor. My guess is that this result could have been due to the small numbers of patients in this category (type 1 error). However, the authors justifiably make the case that if heavy or binge drinkers were to stop or modify this activity in early pregnancy before the second trimester this article would suggest we would not have to worry about this unexpected finding.
Speaking of things to worry about, a few years back we were interested in correlating measurements of certain areas of the fetal brain with maternal alcohol consumption and, then, later, with sophisticated testing of reaction times in the same children. To make a long story short, we found that indirect measurements of the size of the fetal frontal lobe correlated inversely with the amount of alcohol consumed. This, in turn, correlated with how poorly the children performed during the above testing process. However, there was no discernible effect unless the average alcohol consumption exceeded 2.9 drinks per day at the time of entry into the study.2,3
Smoking and alcohol certainly seem to go together, even in pregnant women. For example, the two of us doing the fetal ultrasound measurements in the above study were supposedly blind to which patients were imbibers and which ones were controls, but it became immediately clear who was who, because, often, soon after the "exposed" patients walked in, the ultrasound room smelled like the smoking lounge at Denver International Airport.
At least one-third of those in the "exposed" group in our study were in the restaurant business where it is common to have "a pop or two" before going home. Although most of these individuals stopped after finding out (sometimes late) that they were pregnant, many of them seemed to exude guilt, fueled by all the warnings out there against any exposure to alcohol. Fortunately, most available data suggest that their guilt is likely unfounded, since it appears at this time that to create full-blown fetal alcohol syndrome, or, it seems, to cause even less severe effects on the fetal brain, larger amounts of alcohol would need to be consumed regularly. The Australian study indicates that the same could be said for preterm birth and IUGR. Although smaller amounts of consumed alcohol cannot be completely excluded as having subtle effects on the fetal brain, there is much more evidence out there to indicate that smoking is more detrimental to the fetus than an occasional glass of wine.
1. O'Leary CM, et al. The effect of maternal alcohol consumption on fetal growth and preterm birth. BJOG 2009;116:390-400.
2. Wass TS, et al. The impact of prenatal alcohol exposure on frontal cortex development in utero. Am J Obstet Gynecol 2001;185:737-742.
3. Wass TS, et al. Timing accuracy and variability in children with prenatal exposure to alcohol. Alcohol Clin Exp Res 2002;26:1887-1896.