Emily Osters new book, Expecting Better: Why the Conventional Pregnancy Wisdom Is Wrong-and What You Really Need to Know, has been getting attention for taking on some of the conventional wisdom about what women should do while pregnant. Oster tackles a wide range of controversial topics using her training as an economist to review available data, weigh competing risks, and make informed decisions. But while Oster makes admirable efforts to use scientific data to back up her recommendations, she ironically ends up supporting one of the biggest pregnancy myths of them all: that tuna should not be consumed by expecting mothers.

A recent write-up of the book by Max Nisen at Business Insider highlights this inconsistency in her argument. Nisen writes:

High mercury fish are bad, but many others are not only low in mercury, but high in beneficial omega-3s, which can boost IQ. The effect holds to the point where people that eat tons of fish have smarter kids on average, despite more mercury exposure. Avoiding it all together is a particularly poor outcome.

But immediately after Nisen reports Osters recommendations, he says Oster also recommends pregnant women skip canned tuna.

Thats quite the contradiction.

Eating fish is a net positive for health and comes recommended by the 2010 Dietary Guidelines for Americans. A panel of 13 nutrition experts and physicians reviewed 46 seafood studies and found moms-to-be should eat at least 8 and up to 12 ounces 2-3 servings of seafood per week during pregnancy to boost their babys brain and eye development. In fact, the Dietary Guidelines say that only four species of fish should be avoided by pregnant women altogether: shark, swordfish, king mackerel, and tilefish. Canned light tuna is specifically mentioned as a fish that is low in mercury.

The recommendation to eat more fish is even more important when you realize that the average pregnant woman in the U.S. eats less than 2 ounces of seafood total in a week.

In the introduction to her book Oster writes, When I was pregnant, I pretty quickly learned that there is a lot of information out there about pregnancy, and a lot of recommendations. But neither the information nor the recommendations were all good. The information was of varying quality, and the recommendations were often contradictory and occasionally infuriating.

Unfortunately, much the same could be said about Osters recommendation on fish. By siding with propagandists and fearmongers, and recommending that pregnant women avoid tuna, Oster is advocating contradictory and even infuriating advice. Instead of using her training to get out the facts, she has in part actually bought into the mercury myth.