How Menstruation Works

By: Craig Freudenrich, Ph.D.

Ovulation (approx. Day 15)

The high estrogen levels have signaled the brain that it is time to ovulate.

  • The hypothalamus secretes massive amounts of GnRH, stimulating the anterior pituitary to secrete massive amounts of LH and FSH (mid-cycle LH/FSH peak). This surge lasts only 24 hours.
  • The LH and FSH stimulate enzymes in the follicle. The enzymatic reactions, along with the pressure of the antrum, release the egg from the follicle. The egg travels into the Fallopian tube, where fertilization can take place. The egg can survive for about 12 to 24 hours after ovulation. The remainder of the follicle, the corpus luteum, stays in the ovary. The corpus luteum secretes estrogen and progesterone.
  • The lining of the uterus approaches its thickest and is ready to receive a fertilized egg.
  • The cervical mucus is at its thinnest.

This is the optimum time for fertilization to occur.