The best method for getting pregnant? Have sex.

Sperm Cells Entering Human Egg

I want to have Emily Oster’s publicist. Oster, an economist, recently published the book Expecting Better: Why the Conventional Pregnancy Wisdom Is Wrong-and What You Really Need to Know. Her book has been publicized everywhere, a tribute to her media connections (she has written for The Wall Street Journal) and superb public relations.

I haven’t read the book, yet, but I have been reading Oster’s columns in the online magazine Slate, and have found them to be a somewhat misleading. There are no factual errors, but, as might be expected for an economist writing about obstetrics, she has no clinical judgment, and that dramatically undermines the quality of her advice.

Today’s piece, The Best Method for Getting Pregnant is a good example. Here’s what Oster has to say:

So, how big a deal is it to detect ovulation? Answer: It matters. From very detailed data on couples—including exactly when they had sex and whether they conceived—we know that pregnancy is only possible in the five or six days leading up to and including ovulation. This suggests that you’ve got to get the timing right, and if you do, the odds are pretty good. Pregnancy rates are 30 percent for sex on the day before or day of ovulation, versus 10 percent five days before. No one in the aforementioned study got pregnant having sex more than five days before ovulation.

So should you lay back and leave your pregnancy to chance, or track, measure, and chart? Depends on your time frame, I suppose. But done right, if you want to have the best shot at getting pregnant, shell out for the pee sticks.

Not exactly.

Oster makes a major assumption, one that is untrue. Oster appears to believe that the reason women don’t get pregnant the first month that they try is because they haven’t timed sex to ovulation. That’s wrong. The main reason women don’t get pregnant each month they are trying has nothing to do with ovulation and everything to do with whether the fertilized egg divides properly, implants properly and grows properly.

As a general matter:

50% of women will get pregnant in 4 months of trying
75% will get pregnant in 8 months
90+% will be pregnant within one year

Why doesn’t every woman get pregnant in the first month she tries? Because of the massive wastage inherent in reproduction.

Every woman is born with millions of eggs and each man produces trillions of sperm during a lifetime. Most are never even used. Even when the conditions are right for pregnancy (a sperm meets an ovum) many things can and do go wrong. The ovum could be abnormal, either in function or in its genes, the sperm could be abnormal, either in function or in its genes, or the combination could be genetically abnormal. In any of these cases, the fertilized ovum could fail to divide.

It is possible for the fertilized ovum to divide into the ball of cells known as the blastocyst, but when it gets to the uterus, it fails to implant properly. Or the blastocyst implants, but it dies and is washed out with a menstrual cycle. Or it starts growing and the woman misses her period (and may even have a positive pregnancy test), but then it dies. In that case, she would get her period a few days late. That’s known as a “chemical pregnancy,” meaning a pregnancy that grows to the stage where it releases a tiny amount of pregnancy hormone into the mother’s bloodstream (hence the positive pregnancy test), and then dies.

That doesn’t even take into account that 20% of well established pregnancies will grow for several weeks and then die, resulting in a miscarriage.

So while Oster is technically correct that ovulation predictor kits are the best method for predicting ovulation, she misses two larger points:

1. The timing of sex is rarely the reason why a woman doesn’t get pregnant in a month that she is trying to get pregnant.
2. For women who ovulate regularly, the majority of women, ovulation reliably occurs approximately 14 days before the next period is expected.

Ironically, Oster’s fundamental error, the belief that it is the timing of sex is the reason why a woman doesn’t get pregnant within the first month of trying, is exactly the type of conventional wisdom that she claims she will debunk. But because she fell for the conventional wisdom about fertility, Oster is not reducing anxiety for women, ostensibly the primary motivation for her book; she is raising anxiety. And it points out a fundamental flaw with Oster’s data driven approach.

As anyone who reads this blog knows, I am totally committed to the use of data in making decisions about pregnancy, but data without both a strong foundation in human physiology and excellent clinical judgment can lead people like Oster to make misleading recommendations.

Pregnancy is not a project, requiring micromanagement in order for it to successfully produce a live, healthy baby. Beyond high quality prenatal care, a few basic restrictions (no smoking, no recreational drugs, among others), and hospital birth, there is really not much a woman needs to do, or can do, to reliably produce a healthy baby exactly when she wants one.

What’s the best method for getting pregnant? Have sex regularly, or if not regularly, within the 14-19 days prior to the next expected menstrual period. That’s it.

And if you don’t get pregnant the first month, there is NO NEED to do, or buy or chart anything. Just try again.