The Female Cycle

Monthly bleeding may have been commonplace among nuns, spinsters, and the infertile centuries ago, but it could hardly have been common among the women to whose uteri we all owe our existence. When they were not pregnant, their breast feeding encouraged lactation, which suppressed ovarian cycling. Short (1976) estimates that it may have been uncommon to experience three consecutive menstrual cycles in a lifetime under these conditions. Accordingly, the female endocrine system’s menstrual cycle has not been subjected to selection pressure for its clock-like attributes. In fact, there are diverse clues that some fraction of women are reflex ovulators, not spontaneous cyclers at all (Clark and Zarrow, 1971).1 In a reflex ovulator, mature follicles await rupture by a surge of hormone which is elicited only by sexual stimulation. The ovum then starts its journey down the fallopian tube, and pregnancy (or, less likely, recycling) ensues.