Moreover, definitions of light, moderate and heavy drinking can vary enormously. Investigators who consider light drinking to be up to a drink a day tend to conclude that the practice is more harmful than those who say light drinking means up to one drink a week. (The alcohol content in “one drink” can also differ.) And many studies are based on an “average” number of drinks per time interval, which can lump together dramatically different drinking patterns—for example, imbibing one a week and bingeing on five in one night in a month. And yet, Day says, “there's a lot of literature that shows that binge drinking may have a bigger effect.” One 2009 study, for example, tied a single binge during pregnancy to hyperactivity and attention problems in children. Most studies of periodic light drinking, on the other hand, find that it has no effects.
Teasing Out the Truth
Because individual studies so often conflict, coming to a consensus about how much alcohol—if any—is safe for an expectant mother is a tall order. Nevertheless, one psychologist couple, Sandra and Joseph Jacobson of Wayne State University, has assessed a good deal of the published literature in an attempt to answer the question. They say that no obvious neurobiological deficits have been detected in children whose mothers consume less than one drink a day during pregnancy. Still, the data do not prove that a drink a day in pregnancy is without effect, Sandra Jacobson warns; it could be that researchers have not been looking at the right outcomes or that their tools are too insensitive to pick up any changes.
Gray believes the latter to be true. Although he and his colleagues have found that moderate drinking during pregnancy (which they define as an average of two to six drinks a week) has no overall effect on child IQ at age eight—the kids of the drinking moms in his cohort actually had higher IQs than the abstainers' kids—they did find, using a new genetic technique, some potential for harm.
Humans have multiple genes for proteins that break alcohol down into harmless by-products. Some people have variations in these genes that cause alcohol to linger in their bloodstream. In a November 2012 paper Gray and his colleagues reported that the more changes in these genes children of maternal drinkers have, the lower their IQs are. (The effect is weak; the IQs of children with four genetic differences were only 3.5 points lower than those of kids with two.) More important, however, the gene variations have no effect on the IQ of children born to nondrinking women, suggesting that there is at least a minimal danger to prenatal drinking for certain individuals.
Because of such findings and all the other uncertainties, policy makers are unlikely to ever give the green light to occasional drinking during pregnancy. “We will never, ever, ever know how much is safe for every individual,” says biologist Kathleen K. Sulik of the University of North Carolina at Chapel Hill. What is harmless for a woman of one race, weight, nutritional status and genetic background may be dangerous for another. That said, the literature is reassuring to the many women who socially drank before realizing they were pregnant and to those who, like me, had one celebratory glass.