menstrual cycle

The menstrual cycle is the roughly monthly rhythm by which a woman's body readies itself for a possible pregnancy and, if none occurs, resets to try again. It is best understood as a coordinated conversation between the brain and the ovary, played out as rising and falling hormones that drive predictable changes in the ovary and the uterine lining.

By convention the cycle is counted from the first day of menstrual bleeding and averages about 28 days, though anywhere from roughly 24 to 35 days is common. It has two halves divided by ovulation: the follicular phase, when FSH drives a follicle to grow and estrogen rises, and the luteal phase, when the corpus luteum produces progesterone. The uterine lining mirrors this — proliferating under estrogen, then becoming secretory under progesterone.

The whole cycle is orchestrated by the hypothalamic-pituitary-gonadal axis: pulses of GnRH from the hypothalamus drive LH and FSH from the pituitary, which act on the ovary, whose hormones feed back to the brain. The pivotal event is when high estrogen flips from negative to positive feedback, triggering the LH surge and ovulation. A useful caveat: cycle length varies mainly because the follicular phase varies; the luteal phase is comparatively fixed at about 14 days.