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.
Is drinking while pregnant really that bad? Economist Emily Oster talks with Wendy Bounds about the research behind the most common pregnancy taboos.
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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.
It's well known that frequent heavy drinking or binge drinking can cause fetal alcohol syndrome, with serious long-term consequences for your child. Yet the evidence shows that light drinking is fine.
What's the big deal? If you gain more weight during pregnancy, your baby will—on average—be larger. Babies who are very large are termed "large for gestational age." Babies who are very small are termed "small for gestational age." Babies in both categories are more likely to have complications.
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 food prohibitions are minor compared with the two items that women are most emphatically warned away from consuming during pregnancy: alcohol and coffee.