On the night of my 32nd birthday, my husband and I enjoyed a delicious dinner while on vacation in Orvieto, Italy. To complement my pasta, I drank a single glass of red wine, my first since learning I was pregnant four months earlier. Even now my indulgence that evening inspires periodic pangs of guilt: Did I stunt my son's potential by sipping that Sangiovese?
Nobody questions the notion that heavy drinking during pregnancy is harmful. It can cause facial abnormalities, central nervous system problems and stunted growth. But evidence regarding the effects of light or occasional drinking is mixed. In five epidemiological studies published in 2012, medical psychologist Erik Mortensen of the University of Copenhagen and his colleagues found that five-year-old children born to women who had one to four drinks a week during pregnancy displayed no deficits in general intelligence, attention or other types of higher-order thinking. On the other hand, in 2011 psychiatrist Nancy Day of the University of Pittsburgh and her colleagues reported that teens born to women who averaged more than one drink a week during pregnancy were twice as likely as those born to nondrinkers to have conduct disorder, a condition characterized by theft, deceit or violence.
The truth is hard to discern because research on the issue is fraught with problems. The ideal type of experiment is not ethical: scientists cannot randomly assign one group of women to drink during pregnancy and compare the outcome with those instructed to abstain. As a result, they must compare what happens to women who choose to drink during pregnancy with those who do not, and these women often differ in important ways. All things considered, having an occasional drink during those nine months—say, one or two a week—probably poses little, if any, harm. Still, some experts warn, light or sporadic drinking may have effects we do not know how to measure.
The thorniest problems with this research involve the ways in which women who drink during pregnancy differ from those who do not. A woman who drinks moderately or heavily is more likely to smoke, use drugs and physically abuse her child than a nondrinker, all of which could worsen her son's or daughter's prognosis independent of alcohol. This woman might also have a genetic background predisposing her—and her children—to behavioral problems.
Meanwhile women who drink lightly while pregnant may have protective characteristics. Compared with abstainers, they “are often the more affluent moms, the more educated moms, and the smarter moms as well,” perhaps because they belong to a higher socioeconomic class than teetotalers, says epidemiologist Ron Gray of the National Perinatal Epidemiology Unit at the University of Oxford. This effect could explain why, in a 2010 study, clinical psychologist Monique Robinson and her colleagues at the Telethon Institute for Child Health Research in Perth, Australia, discovered that children of mothers who drank two to six drinks a week while pregnant were less likely to have behavioral problems in the first 14 years of their lives than were children of mothers who abstained. As with most such studies, the researchers tried to account for systematic differences in the groups, but as Robinson acknowledges, “these unmeasured factors may have influenced our results.”
Another complexity: most studies assess maternal drinking through interviews, and pregnant women might lie about or underestimate their consumption out of embarrassment or shame. Lower estimates can mask harmful effects if they cause light drinkers to be incorrectly categorized as abstainers and put in the comparison group. They might, however, inflate the perceived risk if heavy-drinking mothers of children with deficits get incorrectly categorized as light drinkers.