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[Content warning: brief discussion of weight gain; lots of discussion of food in pregnancy.]

There are a lot of books and websites in the genre “Mom Without Medical Background But With Quant Background Does Lit Reviews About Pregnancy.”

If you take the this seriously, a lot of pregnancy advice is useless or actively counterproductive. For instance, pregnant people are regularly advised not to change kitty litter. But– they argue– in reality, toxoplasmosis infections are almost always due to undercooked meat and unprotected contact with soil. Transmission due to cat litter is extraordinarily rare. An infected cat can only transmit toxoplasmosis for two weeks, and it’s unlikely those two weeks would happen while you’re pregnant. And if you change the kitty litter on anything approaching a regular schedule you’ll change it well before you can catch toxoplasmosis by touching poop. (Debunking the Bump, chapter one; Expecting Better, chapter nine.)

Or take weight gain. Pregnant people often worry about gaining too much weight. But gaining a little bit too much weight during pregnancy isn’t correlated with any particularly interesting outcomes, while gaining a little bit too little weight leads to low birth weight, which leads to every negative consequence imaginable. It’s much, much, much better to err on the side of gaining too much weight than too little. (Debunking the Bump, chapter four; Expecting Better, chapter ten.)

Fish consumption is the single most embarrassing example. Many pregnant people limit their fish consumption for fear of mercury poisoning, which lowers IQ. But eating fish in pregnancy is correlated with an increased IQ in children if you eat up to eight servings a week– well above the FDA’s recommended level of fish consumption. This is probably because fish is rich in omega-3 fatty acids. (Debunking the Bump, chapter two; Expecting Better, chapter six.)

And it goes on and on like this! For pages and pages! Which leaves me, a prospective parent, with the question: why is all pregnancy advice so bad?

I have thought of three possible explanations.

First, maybe random mothers with quant backgrounds are an unreliable source of information about pregnancy health. Perhaps I should pay attention to the government, my doctor, and What To Expect When You’re Expecting, instead of women with no medical background and access to Sci-Hub and Google Scholar. But when I’ve spot-checked the books they’ve generally been pretty accurate. And they mostly seem to agree with each other about what the data says, even though as far as I know they aren’t talking to each other. Most of the time, when people are making up random bullshit, they tend to make up different random bullshit (see also: nutrition advice). Also, I don’t like this explanation because Emily Oster says I’m allowed to eat sushi and What To Expect says I’m not and I don’t want to go through my nine mouths of pescetarianism sushiless.

Second, maybe all medical advice is like this. The general quality of information about medicine in general is exactly the same as the general quality of information about pregnancy, it’s just that I’ve bothered to actually look into pregnancy. This is very worrisome. What if this whole time I’ve been exercising because I thought it would make me live longer and actually I’m shortening my lifespan? What if I have been trying to cut down on neon-colored high-fructose-corn-syrup snacks for absolutely no reason? What if the true secret to fixing depression was doing handstands every day to encourage blood flow to the brain, and all the “fix your depression with yoga” people have been right all along?

Of course, the very most important pregnancy advice is generally accurate: it is really really bad to smoke or to drink to excess, and it’s much better to have a pregnancy before 35 than after 35. And a lot of the research is relatively new. We only started figuring out how pregnant people got toxoplasma gondii in 2000, and before then it was “like idk cats probably?” The FDA found that fish consumption was correlated with higher IQ in 2009. Folic acid was really really important before mandatory food fortification began in 1998, and it took a while to realize that very few people are folic acid deficient anymore. Perhaps it takes a while for this information to filter down to the general public, and in thirty years pregnant people will devour two plates of salmon and then go change the cat litter. So maybe the take home here is that medical consensus is correct about really really important things, but tends to be slow to adopt new information. (Which might be a good thing! There is, after all, a replication crisis.)

Third, maybe pregnancy advice is unusually bad for some reason. For instance, people do seem to be unusually risk-averse about pregnant people compared to other groups. So maybe people come up with recommendations (“don’t eat fish! don’t change cat litter!”) based on relatively weak evidence, because what if a bunch of women get mercury poisoning and then we have a huge epidemic of mercury babies? And when it turns out that cat litter or fish is actually fine, people tend to keep giving advice: what if the study was wrong? They want to err on the side of caution.

I think that risk aversion plays a nonzero role: for instance, I expect that’s behind folic acid supplementation (best to be on the safe side about whether the pregnant person is eating enough Cheerios) and telling women to avoid sushi (parasite infections from sushi are very rare but they do happen). And it probably even explains some of the weird downplaying of risk: if pregnant people are told to avoid both unpasteurized cheese and deli meat, without a clear explanation that deli meat is far far more dangerous than unpasteurized cheese, they might forget that they’re not supposed to be eating deli meat. But I’m not sure that risk aversion fully explains the fish thing or the weight gain thing.

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