by EMILY OSTER
When I got pregnant three years ago, I knew that I’d face lots of decisions—whether to have coffee or a glass of wine, what kind of prenatal testing to do, whether to use an epidural. I figured that I’d study the pros and cons and then make my own informed choices, as I normally did (with guidance and information from my doctor, of course).
This isn’t what it was like at all.
In reality, medical care during pregnancy seemed to be one long list of rules. Being pregnant was a good deal like being a child again. There was always someone telling me what to do, but the recommendations from books and medical associations were vague and sometimes contradictory. It started right away. “You can only have two cups of coffee a day.” I wondered why. What did the numbers say about how risky one, two or three cups were? This wasn’t discussed anywhere.
The key to good decision making is evaluating the available information—the data—and combining it with your own estimates of pluses and minuses. As an economist, I do this every day. It turns out, however, that this kind of training isn’t really done much in medical schools. Medical school tends to focus much more, appropriately, on the mechanics of being a doctor.
When I asked my doctor about drinking wine, she said that one or two glasses a week was “probably fine.” But “probably fine” isn’t a number. In search of real answers, I combed through hundreds of studies—the ones that the recommendations were based on—to get to the good data. This is where another part of my training as an economist came in: I knew enough to read the numbers correctly. What I found was surprising.
The key problem lies in separating correlation from causation. The claim that you should stop having coffee while pregnant, for instance, is based on causal reasoning: If you change nothing else, you’ll be less likely to have a miscarriage if you drink less coffee. But what we see in the data is only a correlation—the women who drink coffee are more likely to miscarry. There are also many other differences between women who drink coffee and those who don’t, differences that could themselves be responsible for the differences in miscarriage rates.
This problem is partially surmountable with better data, and I found that the best studies often painted a picture different from the official recommendations. Actually getting the numbers led me to a more relaxed place—a glass of wine every now and then, plenty of coffee, exercise when I wanted it. The economist’s toolbox may not be known as a great stress reliever, but in this case it really was.
Consider weight gain. Almost every woman I have spoken to about pregnancy has a story about her doctor giving her a hard time about her weight. Later in my pregnancy it felt like all of my time with my doctor was focused on how fat I was getting—so fat! I was supposed to gain between 25 to 35 pounds. At one visit it seemed like I was on track for 36 pounds, and I got a serious scolding.
Weight-gain guidelines are designed to maximize the chance that your baby is normal sized. If you gain less weight than recommended, you increase your chance of a small baby and decrease your chance of a large baby; vice versa if you gain more than recommended.
There is a nice logic to this, but I quickly realized that very small babies are associated with many more—and more serious—complications than very large babies. The main concern with a very large baby is difficulty in delivery. Very small babies have an increased risk of breathing problems and neurological complications.
The recommendations—which focus only on the probability of each thing happening and not on the magnitude of the problem—are incomplete. In the end, I concluded that I should be more worried about gaining too little weight than too much.
Pregnant women are also given a long list of off-limit foods: deli meats, soft cheeses, sushi. These are restricted because of the risk of various pathogens. The most serious by far is listeria bacteria, to which pregnant women are especially susceptible; it can cause late miscarriage and stillbirth.
I knew I didn’t want to snack directly on listeria bacteria, but I wondered how much I could limit my risk by avoiding certain foods. What share of listeria infections was due to soft cheeses, for instance? It turns out that queso fresco, a Mexican soft cheese, has been implicated in about 20% of listeria outbreaks since 1998, and deli turkey in 10%. The rest of the recent outbreaks seemed random. One involved cantaloupe, another one, celery.
I concluded that avoiding queso fresco and deli turkey was a good idea, but in the end I didn’t feel that it made sense even to exclude other deli meats. My best guess was that avoiding sliced ham would lower my risk of listeria from 1 in 8,333 to 1 in 8,255. I just didn’t think it was worth it. It would have made more sense to avoid cantaloupe.
But all drinking isn’t created equal. When I looked at the data from hundreds of studies, I found, basically, no credible evidence that low levels of drinking (a glass of wine or so a day) have any impact on your baby’s cognitive development.
To learn about the impact of light drinking on pregnancy, we actually want to look at women who are light drinkers. I started scouring the medical literature for studies that focused on this group—that is, women having up to a drink a day. A problem in all this research is that the kinds of women who drink are different from those who don’t. The key was to find the studies that had minimized this problem.
One big worry about drinking during pregnancy is that it will result in child behavior problems later. One of the best studies of this issue was published in 2010 in the British Journal of Obstetrics and Gynecology. What makes it a reliable study? The sample group was large (3,000 women), and the researchers collected information about maternal drinking during pregnancy—not afterward. The study also followed the children of these women through the age of 14 and looked at behavior problems starting at age 2.
The other thing I liked about this study was that it was run in Australia, where recommendations on drinking during pregnancy are more lax than in the U.S. Because the rules are more permissive, Australian women who drink occasionally aren’t necessarily the kind of women who go against medical advice; it’s more likely that differences in drinking levels there are just random variation. Drinkers in the study were classified in five groups: no alcohol, occasional drinking (up to one drink a week), light drinking (2-6 drinks a week) and moderate drinking (7-10 drinks a week).
The researchers compared the mothers’ drinking level at 18 weeks of pregnancy with the children’s behavior issues at age 2. They found that 11% of the children whose mothers did not drink during pregnancy had behavior problems—versus 9% of the children of light drinkers and 11% of the children of moderate drinkers. (Nearly 14% of 2-year-olds whose mothers occasionally drank had behavior problems, but the difference is small and, statistically, could have occurred by chance.) The results were very similar for older kids.
The other big concern with alcohol is low IQ. Again, my favorite study on this issue comes from Australia. It gave IQ tests at age 14 to the children of 5,000 women who drank at different levels during pregnancy. They measured IQ with a test called Raven’s. It works like most IQ tests (higher scores are better), and the test is designed so that the average person will score 100. Children of women who had an occasional drink during early pregnancy scored an average of three points higher then children of women who abstained. The results were similar for women in late pregnancy. In short, just as in the study that focused on behavior problems, there is no evidence here to suggest that the children of light drinkers are worse off than those of women who don’t drink at all.
It is possible to unearth research that points to light drinking as a problem, but this work is deeply flawed. One frequently cited study from the journal Pediatrics, published in 2001, interviewed women about their drinking while they were pregnant and then contacted them for a child behavior assessment when their children were about 6. The researchers found some evidence that lighter drinking had an impact on behavior and concluded that even one drink a day could cause behavior problems.
In the study, 18% of the women who didn’t drink at all and 45% of the women who had one drink a day reported using cocaine during pregnancy. Presumably your first thought is, really? Cocaine? Perhaps the problem is that cocaine, not the occasional glass of Chardonnay, makes your child more likely to have behavior problems.
The evidence overwhelmingly shows that light drinking is fine. Of course, this is sensitive to timing. Both the data and the science suggest that the speed of drinking, and whether you are eating at the same time, matters. It isn’t that complicated: Drink like a European adult, not like a fraternity brother.
A far bigger issue for me was coffee. I love coffee. The thought of giving it up during pregnancy struck fear into my heart. Of course, for the baby, I’d do anything. But I didn’t want to do it for no reason.
The big concern with consuming caffeine during pregnancy is that it might lead to higher rates of miscarriage. Caffeine can cross the placenta, entering the fetus’s bloodstream, and it isn’t clear how the fetus processes it. In addition, researchers have speculated that caffeine can inhibit fetal development by limiting blood flow to the placenta.
Still, these effects have not been proven. In the end, randomized experiments are difficult or impossible, and women who drink coffee tend to be different from those who don’t. One big issue is that older women tend to drink more coffee, and age and miscarriage are closely linked.
Studies of the impact of caffeine on miscarriage have another problem: nausea. Nausea is a normal but unpleasant effect of pregnancy and a really good sign that it is going well. Women who experience nausea in early pregnancy are less likely to miscarry.
My morning routine, when not pregnant, is to have a cup of coffee before breakfast on an empty stomach. Early in my pregnancy, this idea was, frankly, revolting. After talking with other women, it sounds like this is fairly typical.
We know that nausea is a sign of a healthy pregnancy, but (as in my case) it also causes women to avoid coffee. This means that the pregnant women who drink a lot of coffee also are more likely to be the ones who aren’t experiencing nausea. So here we may well be mistaking a correlation for an underlying cause: The women who drink less coffee have fewer problems not because they limit their caffeine intake but because they tend to suffer from nausea, which inhibits coffee drinking.
Studies that take into account the nausea issue suggest that caffeine during pregnancy is fine, in moderation. In an attempt to support my three- to four-cups-a-day habit, I investigated dozens of studies, some of which were pretty good. Consider one covering about 2,400 women, published in the journal Epidemiology in 2008. Women who consumed more than two cups of coffee a day at 16 weeks of pregnancy had, if anything, lower rates of miscarriage by 20 weeks than those who consumed none (although this difference wasn’t statistically significant).
But not every credible study is so encouraging. One published in 2008 in the American Journal of Obstetrics and Gynecology found no difference in miscarriage rates between women who drank no coffee and those who drank up to two cups a day. But the researchers did find higher miscarriage rates for those who drank more than two cups a day. The differences in this study are big: a 25% miscarriage rate for those women who drank more than two cups a day, versus only about 13% for those who drank less.
Concerning, yes. But some things in this study gave me pause. For one thing, the authors found no effect from coffee among women who reduced their consumption during pregnancy, regardless of what their final consumption level was. Taken literally, this means that it doesn’t matter how much coffee you drink each day, as long as you consume less than your pre-pregnancy level. It’s hard to figure out why this might be, other than that women who are nauseous reduce their consumption.
There were a few other things that made me think the nausea story might be important. One was that other common sources of caffeine—tea and cola—are less consistently linked with miscarriage. These contain caffeine but tend to be easier on the stomach, so the confounding relationship with nausea is limited.
I’m hardly alone. Pregnant women are clamoring for better information about everything from exercise to hair dye to bed rest and delivery. They don’t want categorical limits based on fuzzy science and half-baked research. They want to assess risks for themselves and make their own best decisions.
—Ms. Oster is an associate professor of economics at the Booth School of Business at the University of Chicago and writes the Ask Emily column on WSJ.com. This essay is adapted from “Expecting Better: Why the Conventional Pregnancy Wisdom is Wrong—and What You Really Need to Know,” to be published Aug. 20 by the Penguin Press. Find her on Facebook at fb.me/profemilyosterA version of this article appeared August 10, 2013, on page C1 in the U.S. edition of The Wall Street Journal, with the headline: Take Back Your Pregnancy.