Ovulation

The preovulatory follicle, through the elaboration of estradiol, provides its own ovulatory stimulus. Considerable variation in timing exists from cycle to cycle, even in the same woman. A reasonable and accurate estimate places ovulation approximately 10-12 hours after the LH peak and 24-36 hours after peak estradiol levels are attained. The onset of the LH surge appears to be the most reliable indicator of impending ovulation, occurring 34-36 hours prior to follicle rupture. A threshold of LH concentration must be maintained for 14-27 hours in order for full maturation of the oocyte to occur. Usually the LH surge lasts 48-50 hours.

The LH surge tends to occur at approximately 3 A.M., beginning between midnight and 8:00 A.M. in over two-thirds of women. Ovulation occurs primarily in the morning during Spring, and primarily in the evening during Autumn and Winter. From July to February in the Northern Hemisphere, about 90% of women ovulate between 4 and 7 P.M.; during Spring, 50% of women ovulate between midnight and 11 A.M.

The gonadotropin surge stimulates a large collection of events that lead to ovulation, the physical release of the oocyte and its cumulus mass of granulosa cells. This is not an explosive event; therefore, a complex series of changes must occur which cause the final maturation of the oocyte and the decomposition of the collagenous layer of the follicular wall.

Prostaglandins may contract smooth muscle cells that have been identified in the ovary, thereby aiding the extrusion of the oocyte-cumulus cell mass. This role of prostaglandins is so well demonstrated that infertility patients are advised to avoid the use of drugs that inhibit prostaglandin synthesis.

An adequate gonadotropin surge does not ensure ovulation. The follicle must be at the appropriate stage of maturity in order for it to respond to the ovulating stimulus. (Source: Principles and Practice of Assisted Human Reproduction. Edwards and Brody. W.B Saunders Company, 1995)