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.