Menstrual Ovulation 

During the follicular phase, estradiol suppresses production of luteinizing hormone from the anterior pituitary gland. When the egg has nearly matured, levels of estradiol reach a threshold above which they stimulate production of LH. These opposite responses of LH to estradiol might be enabled by the presence of 2 different estrogen receptors in the hypothalamus - estrogen receptor alpha, which is responsible for the negative feedback estradiol-LH loop, & estrogen receptor beta, which is responsible for the positive estradiol-LH relationship. In the average cycle this LH surge starts around cycle day 12 & might last 48 hours.

The release of LH matures the egg & weakens the wall of the follicle in the ovary, causing the fully developed follicle to release its secondary oocyte. The secondary oocyte promptly matures into an ootid & then becomes a mature ovum. The mature ovum has a diameter of about 0.2 mm.

Which of the 2 ovaries—left or right—ovulates appears essentially random no known left & right co-ordination exists. Occasionally, both ovaries will release an egg if both eggs are fertilized, the result is fraternal twins.

After being released from the ovary & into the peritoneal space, the egg is swept into the fallopian tube by the fimbria, which is a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg will disintegrate or dissolve in the fallopian tube.

Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest section of the fallopian tubes. A fertilized egg immediately begins the process of embryogenesis, or development. The developing embryo takes about 3 days to reach the uterus & another 3 days to implant into the endometrium. It has usually reached the blastocyst stage at the time of implantation.

In some women, ovulation features a characteristic pain called mittelschmerz . The sudden change in hormones at the time of ovulation sometimes also causes light mid-cycle blood flow.