The Real Reason Why the Rhythm Method Doesn't Work?

For women trying not to get pregnant, life should be easy. Conception can only happen 12-24 hours after ovulation. Sure, sperm may last as many as 3-4 days in the genital tract, hanging around for the egg to arrive. But you’d think that, to avoid a pregnancy, all you'd have to do is to abstain from sex for the 4-5 days around the ovulation window.

That’s what the rhythm method is — a natural form of birth control that relies on abstinence during a woman's fertile days.

But pregnancies happen even among the most careful practitioners. Some of this can be attributed to human error -- women not keeping perfect track of their menstrual cycles or having naturally irregular cycles. The failure rate for rhythm method is 25 percent each year (with perfect use the rate is still nearly 10 percent).

This is shockingly high. Why so high?

The hidden reason could be pheromones, chemical signals that subtly influence our behavior without our knowing. It's just speculation in the journal Medical Hypotheses, but it's worth mentioning. The submission suggests that pheromones that men put out in their sweat and saliva may trigger early ovulation in women. The phenomenon has been observed in other studies, including this one at the Monell Chemical Senses Center. The early release of the egg — in advance of the expected fertile window — increases the chances of fertilization.

The target chemical is androstadienone, a testosterone-related compound. It's not only in men’s sweat, but also in their semen and saliva. Androstadienone works its charm by increasing the amount of luteinizing hormone in women, which thereby triggers ovulation. Women inhale the chemical in men's sweat (or absorb it orally or vaginally), whereby it acts on their hypothalamus, the region of the brain that controls hormone secretion.

It’s possible that high-testosterone men -- who produce more androstadienone -- may be likelier tham low-testosterone men to have an accelerated-egg-release-effect on their lovers. Their sweat alone may do the trick.

As I mentioned in a previous post, there are other properties in semen that may also trigger early ovulation. For instance, seminal fluid contains follicle stimulating hormone (FSH) and luteinizing hormone (LH), which may coax the ovary to release an egg.

Bottom line: Fertility is unpredictable -- and many events may throw us out of sync. We don’t live in a clockwork universe, nor do we have clockwork bodies.

Note: A previous version of this post mentioned that the rhythm method is part of natural family planning (NFP), a term used by the Roman Catholic Church. NFP includes cervical mucus, temperature data, and other forms of fertility monitoring that are much more accurate than the rhythm method.

*If you like this blog, click here for previous posts. If you wish, check out my new book, Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.