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Is light drinking while pregnant okay? Here's what the evidence says.

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This week, the Centers for Disease Control and Prevention came out with a new campaign advising all sexually active women of childbearing age who aren't using contraception to avoid alcohol. The logic here, apparently, is that these women might get pregnant, and any amount of alcohol could damage the fetus.

"The risk is real," Anne Schuchat, principal deputy director of the CDC, said in a statement. "Why take the chance?"

The backlash was swift. Critics called the advice puritanical, paternalistic, and overly cautious. "It’s the kind of swath-yourself-in-bubble-wrap thinking that has turned modern pregnancy into a nine-month slog of joyless paranoia," Ruth Graham wrote in Slate. And, indeed, there were many things wrong with the CDC's crude messaging. (Aaron Carroll does a great job of summing them up.)

One question that hasn't gotten nearly as much scrutiny, however, is how solid the science behind the CDC's advice is. Everyone agrees that heavy drinking during pregnancy can be harmful. But does light or moderate drinking in the early stages of pregnancy actually carry any negative effects? The medical community hasn't been able to agree on a safe level of alcohol in pregnancy, which is why many public health agencies have taken a zero-risk approach telling women to abstain.

Many people are skeptical. Emily Oster's widely read book Expecting Better concludes there's little evidence that light drinking causes harm. More specifically, she wrote, there's little good science to suggest that up to one to two drinks per week during the first trimester, and up to one drink a day during the second and third trimesters, harms babies. Oster's book, which was published in 2013, has been widely touted by those criticizing the CDC.

But that's actually not the end of the story. Since Oster's book was published, newer evidence has emerged that has complicated this narrative — and suggests that even light drinking during pregnancy could have unexpected adverse impacts. This is still an area of scientific uncertainty, so let's wade through the evidence.

The key question: Can even a small amount of alcohol during pregnancy be harmful?

One thing that makes this question tricky is that it can be incredibly difficult to study. For obvious reasons, no research body would grant ethical approval to run a randomized controlled trial in which some pregnant women are told to drink alcohol in order to see what happens to their babies.

That means many of the studies we're left with are observational: Scientists look at women who already drink a little or a lot during pregnancy and women who don't — and then compare the health of their children, measuring IQ at the age of 8, or behavior problems, or levels of employment later in life.

These observational studies have found that children whose mothers drank lightly (say, a glass a week or less) during pregnancy often do better than the children of abstainers.

But there's a problem here. In many countries (including the US), the women who drink moderately while pregnant tend to be better educated and wealthier than those who don't — factors that would influence their offspring's intelligence and health. And even the best studies can't completely control for that difference. So it's possible these socioeconomic factors are the reason for any variation in children's outcomes, not the alcohol itself.

Newer research has found possible problems with light drinking during pregnancy

To get around the flaws in earlier studies, a group of researchers at Bristol University in the UK wanted to try a different approach. They built several interesting studies by following pregnant women recruited for a massive research program in the Bristol area that involves 14,000 families.

Instead of grouping women based on how much they reported drinking during pregnancy, in a 2013 study, the researchers used a technique known Mendelian randomization. This involved grouping pregnant women based on their genetic predisposition to drink — that is, whether they carried a gene variant that's associated with reduced alcohol consumption as consequence of faster alcohol metabolism. In layman's terms, they wanted to find women who reacted badly to alcohol and therefore avoided it.

What the researchers found was that women with a low tolerance for alcohol were more likely to abstain or drink less. And, interestingly, their children performed better on IQ tests and school performance measures compared with children of women who better tolerated alcohol.

One of the authors of that study, Luisa Zuccolo, explained, "As a consequence of their genotype, these women tend to drink a bit less or not at all — and we found their babies did better in IQ tests and school results."

In another 2012 study, the researchers came to a similar conclusion. They looked at genetic variants related to metabolizing alcohol in more than 4,000 kids. The kids who carried the genetic mutations — and who came from moms who drank between one and six units a week of alcohol — actually had lower IQs (about 2 IQ points per genetic variant) compared with those who did not. The researchers say this suggests that even moderate amounts of alcohol during pregnancy could have an effect on a child's future intelligence.

"Until now, people haven't been able to categorically prove that very low levels of alcohol cause harm," said Bristol genetic epidemiologist Sarah Lewis, the lead author on the study. "But using new techniques, more and more studies are suggesting low levels might have an impact."

The researchers behind these studies noted that more research on this question needs to be done to know whether their results are real. Still, this line of research points to potential genetic components that might put some babies at a higher risk of harm from even a little bit of alcohol.

The new research is part of the reason the UK's chief medical officer, in the beginning of January, released new guidelines about drinking that suggested that pregnant women should abstain completely. (The UK's previous guidelines had suggested light alcohol while pregnant was okay.)

Before you freak out...

These studies are relatively new. They need to be repeated in different settings and populations to see if the conclusions hold. "This seems like a space where replication is crucial," Emily Oster wrote in an email. "I hope these authors plan to do more with these genetic links to try to get at the details of this relationship."

The UK's decision was based on the precautionary principle. It can't say definitively that a little booze causes harm, but there's some evidence that it might. So out of an abundance of caution, the UK suggested women simply avoid drinking while pregnant.

It's also important to note that in these new studies, the effects the researchers found weren't large. No one is suggesting a single drink during pregnancy will cause fetal alcohol syndrome or severely diminished IQ. At most, the research suggests that examining drinking during pregnancy in new ways — based on genetic predisposition — might reveal subtle differences that no one's noticed before.

There are still a lot of unknowns about the effects of alcohol on the fetus — such as why some women can drink quite a lot during pregnancy with little effect on their children while others can't. But for now, the preponderance of evidence suggests light drinking during pregnancy probably isn't harmful. As the evidence evolves, however, that conclusion might change.