For women not trying to get pregnant, life should be easy. Conception can only happen in the 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 not having sex during the 4-5-day window would be sufficient to avoid mishaps. That’s what the rhythm method is — a natural form of birth control that relies on abstinence on fertile days.

But slips happen even among the most careful practitioners of the rhythm method. Some of this may have to do with women not keeping perfect track of their menstrual cycles or having naturally irregular cycles. (I discuss in BLONDES the evolutionary reasons why ovulation is hidden to both women and their partners.) The failure rate for rhythm method is 25 percent each year (with a perfect-use rate of 9 percent).

Why so high?

Another reason could be pheromones. The latest issue of my favorite journal, Medical Hypotheses, includes a submission that suggests that pheromones from men may cause an early ovulation in women. By invoking an early release of the egg — in advance of the expected fertile window — chances of fertilization are higher. As I mention in BLONDES, studies have the found that androstadienone, a testosterone-related compound found in men’s sweat, semen, and saliva, increases the amount of luteinizing hormone in women, which thereby triggering ovulation. It’s possible that high-testosterone men may be likelier to have this effect on their lovers. Their sweat smell alone may do the trick.

As I mentioned in an earlier 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.

Despite the high failure rate, the Roman Catholic Chruch continues to promote the rhythm method, now renamed natural family planning (adding cervical mucus and temperature data to the regimen). Problem is, we don’t live in a clockwork universe, nor do we have clockwork bodies.