ovulation

Ovulation is the release of a mature egg from the ovary — the single dramatic event at the center of the menstrual cycle and the moment that makes pregnancy possible. After two weeks of preparation, the dominant follicle bursts open and the egg is swept into the fallopian tube, where it can meet sperm.

Ovulation is triggered by the LH surge: a sudden, sharp spike of luteinizing hormone from the pituitary, itself caused by sustained high estrogen flipping into positive feedback. The surge sets off a cascade of changes in the follicle — enzymes weaken its wall, fluid pressure builds, and the egg completes a key step of its maturation — culminating in rupture and egg release roughly 24 to 36 hours after the surge begins.

After releasing the egg, the collapsed follicle becomes the corpus luteum and the luteal phase begins. The egg itself remains fertilizable for only about 12 to 24 hours, but because sperm can survive several days, the fertile window spans a few days before ovulation. A practical caveat: ovulation timing is variable and can be shifted or suppressed by stress, illness, or hormonal contraception, so cycle-day counting alone is an unreliable guide to exactly when it happens.