

It happened again to my friend C. — she had an affair with a dashing Moroccan guy two weeks ago and got her period early. She always experiences short or untimely cycles in the beginning of relationships, and do many women I know. One study found that one out of every three gals reports getting her period early in response to a new sex partner, or — as been found among Army wives — a visit from an existing one who lives far away. Even among women whose cycles are like clockwork, novel sex can make them cuckoo.

There are many factors that can throw off a cycle, including emotional excitement and stress. As I discuss in BLONDES, there’s evidence that sweat and semen can, too. Digging deeper into the psychobiology of semen, I stumbled upon a provocative theory. It all begins with the discovery that semen contains “female hormones” including follicle stimulating hormone (FSH) and luteinizing hormone (LH).

Why are these so-called female hormones in semen? The answer, according evolutionary psychologists Gordon Gallup and Rebecca Burch at SUNY Albany, is that they increase the man’s chances of impregnating the woman. Unlike females of other species, women don’t advertise when they’re ovulating. And because ovulation is concealed and the fertile window easy to miss, it may take many months of regular sex for conception to happen.

But what would happen if men could control the timing?

Turns out that hormones in semen may do exactly that — by mimicking the conditions of ovulation. The FSH in semen causes an egg in an ovary to ripen and mature. The LH in semen triggers ovulation and the release of the egg. If a woman is near ovulation — say, in the first week or so of her cycle — these seminal chemicals may be sufficient to induce an early release of the egg. By synchronizing ovulation with sex, a man improves his chances of conception in a casual encounter.

As a reason why female hormones are present in human sperm, the theory sounds plausible. Women conceal ovulation to prevent men from knowing when they’re fertile (for reasons I discuss in the book). Men, in an evolutionary tit for tat, developed ways to induce ovulation to increase the chances of conception. Indeed, when researchers analyzed the sperm of chimpanzees, which advertise ovulation rather than conceal it, levels of the female hormones LH and FSH were low to nonexistent.

Of course, questions remain: How do women’s cycles normalize a few months into the relationship despite ovulation-inducing hormones? Are FSH and LH hormones exclusively “female;” what other purposes might they have for men?

As for my friend C., she didn’t get pregnant from her fling. Fortunately, her cycle only accelerated. But the risk of a surprise pregnancy is one more reason why it’s a good idea to use a condom.

