The peak fertility for women is around age 27.
This level stays about the same until approximately age 37.5, when it begins to decline. At age forty, the rate of decline accelerates and by age 42 a woman has lost almost 50% of her fertility. This decline continues until the menopause, which occurs around age 51 (range: 40-55 yrs) in the U.S.
Unlike men who continue to produce sperm throughout their lives, women are born with all the eggs they are ever going to have. At the time of puberty a woman has about 500,000 eggs and she looses them at a rate of approximately 1000 per month. Of those 1000 eggs, only one will mature enough to ovulate. The others will atrophy and die off.
The development of eggs is controlled by the pituitary gland which releases a hormone called Follicle Stimulating Hormone (FSH). FSH signals the ovary to make a group of follicles grow. Inside of each follicle is an egg. The follicle produces estrogen, which signals the brain that the system is working. At the beginning of a woman’s reproductive life-span, the ovary responses readily to stimulation from the brain to produce a mature egg. However, over time the ovary uses up its best eggs. As with a barrel of apples, at some point there are only the bad ones left at the bottom. These remaining eggs do not respond as easily to the FSH signal from the brain. The brain then makes more FSH in an attempt to coax the ovary to produce an egg.
We often measure the FSH level on the third day of the menstrual cycle as an indication of ovarian reserve. When the FSH level rises above 9 mIU/ml it indicates the onset of ovarian resistance to fertility drugs. An FSH of above 12 mIU/ml (as measured in our laboratory), points to a degree of ovarian resistance that is associated with a markedly reduced chance of having a successful pregnancy using the woman’s own eggs, even with IVF.
An elevated FSH level does not necessarily mean that the woman is entering menopause early, (although it can), only that her reproductive potential may be reduced. Most women with increased FSH levels continue to have regular periods. Women with elevated FSH levels do not respond to stimulation with fertility medications and are best treated using donor eggs.