Ovarian physiology

Egg development

Women receive their entire lifetime supply of eggs prior to their birth and soon begin the steady process of losing them. Starting from a maximum number of 7 million eggs, women have approximately 300,000 when going through puberty, and may have only a few thousand remaining by their late thirties. Eventually a woman exhausts her supply of eggs, ceases having spontaneous menstrual cycles, and enters menopause.

Ovulatory cycles

A typical woman ovulates approximately 400 times in her life (i.e. 13 cycles per year between puberty and menopause).

How ovulatory cycles work:

• Eggs rest in the ovary and are surrounded by special cells responsible for supporting the developing egg and for making the various hormones associated with ovulatory cycles. The combination of the egg and its special supporting cells is called a follicle.
  
  
• Anywhere from 50 to several hundred follicles typically leave this resting state each month. They must either mature and release the egg or stop growing. As they grow, they become dependent on the stimulation of hormones from the pituitary gland to continue their development. These hormones are called follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

• Follicles begin producing their own hormones such as estrogen. The relationship between the hormone production of the developing follicles growing in the ovary and the stimulating hormones from the pituitary gland creates a mechanism that controls the number of follicles (eggs) which complete the maturation process each cycle.
  
  
• Precise control of circulating levels of FSH allows only one of the follicles to develop to maturity. This is why women typically become pregnant with one baby at a time.