Fertility and the full moon

By Allen J. Wilcox

On making boy babies, and other pregnancy myths

In her novel, Prodigal Summer, Barbara Kingsolver celebrates the lush fecundity of nature. The main character marvels at the way her ovulation dependably comes with the full moon.

It’s a poetic image – but is there any evidence for it?

Actually, no. It’s true that the length of the average menstrual cycle is close to the length of the lunar cycle. But like so many notions about fertility, an effect of the moon on ovulation is just a nice story. The menstrual cycle is remarkably variable, even among women who say their cycles are “regular.” This is not surprising – unlike the movement of stars and planets, biology is full of variation. The day of ovulation is unpredictable, and there is no evidence (even in remote tribal cultures) that ovulation is related to phases of the moon or other outside events.

We humans are susceptible to myths about our fertility and pregnancy. These myths also invade science. One scientific “fact” you may have heard is that women who live in close quarters synchronize their menstrual cycles. The paper that launched this idea was published forty years ago in the prestigious journal Nature¹. Efforts to replicate those findings have been wobbly at best – but the idea still persists.

Another scientific myth is the notion that sperm carrying the Y male chromosome swim faster than sperm carrying the X female chromosome. It’s true that the Y chromosome is smaller than the X.  But there is no evidence that this very small addition of genetic cargo slows down the X-carrying sperm. As often as this idea is debunked, it continues to appear in scientific literature – and especially the literature suggesting that couples can tilt the odds towards having a baby of a particular sex.

Choosing your baby’s sex

Many couples have a definite preference for the sex of their baby. The baby’s sex is established at conception, which has led to a lot of advice on things to do around the time of conception to favor one sex or the other.  Recommendations include advice on timing of sex in relation to ovulation, position during sex, frequency of sex, foods to eat or avoid, etc. The good thing about every one of these techniques is that they work 50% of the time. (This is good enough to produce many sincere on-line testimonials.) Despite what you may read, there is no scientific evidence that any of these methods improves your chances for one sex or the other, even slightly. The solution? Relax and enjoy what you get.

When will the baby arrive?

Everyone knows that pregnancies last nine months – but do they? Doctors routinely assign pregnant women a “due-date,” estimated from the day of her last menstrual period before getting pregnant. The due-date is set at 40 weeks after the last menstrual period. You might think the due-date is based on scientific evidence, but in fact, 40 weeks was proposed in 1709 for a rather flaky reason: since the average menstrual period is four weeks, it seemed “harmonious” for pregnancy to last the equivalent of ten menstrual cycles.

So what are a woman’s chances of actually delivering on her due date?  Fifty percent? Twenty percent?

Try four percent. Just like the length of menstrual cycles (and every other aspect of human biology), there is lots of variation in the natural length of pregnancy. If the due-date is useful at all, it is as the median length of pregnancy – in other words, about half of women will deliver before their due-date, and about half after. So don’t cancel your appointments on the due-date just because you think it’s The Day – there’s a 96% chance the baby will arrive some other time.

1. McClintock MK. Menstrual synchorony and suppression. Nature 1971; 229:244-5.

Allen J. Wilcox, MD, PhD is Senior Investigator at the National Institute of Environmental Health Sciences (NIH) in Durham, NC, and Editor-in-Chief of Epidemiology. He is a pioneer in reproductive epidemiology, with research projects on topics ranging from fertility and early pregnancy loss to fetal growth and birth defects. Dr. Wilcox is author of Fertility and Pregnancy: An Epidemiologic Perspective.

View more about this book on the