ABSTRACT
In common with the females of most other mammalian species, women experience regularly recurring cycles in their ability to conceive and establish a pregnancy, loosely called "menstrual cycles" in humans because periodic menstrual bleeding is their most conspicuous feature. Although many tissues in the body participate in these cycles, including parts of the brain and uterus, the principal actor in many respects is the ovary. From a biometric point of view, components of the ovarian cycle can induce variation in fertility only insofar as they vary themselves. This chapter explores the variation in cycles and their components, and the possible contribution of such variation to fertility differentials. It outlines the basic physiology of the ovarian cycle. Several studies have shown that the probability of a cycle being an ovulatory or having an inadequate luteal phase is highest during the first few years following menarche, with a secondary rise during the five or so years preceding menopause.
