Although age is not an absolute barrier to pregnancy, female fertility decreases with age. Women are born with a fixed number of eggs -- about 400,000 eggs. This number decreases even during early childhood, bringing the number closer to 300,000 eggs by the time a woman reaches puberty. Over the next 35 to 40 years of reproductive life, only 400 to 500 eggs will ovulate and at menopause only a few hundred will remain. In particular, there is an acceleration of follicular loss during the last 10 to 15 years before menopause.

This loss is associated with a small increase in follicle stimulating hormone (FSH), and reflects the reduced quality and capability of the eggs. Ovarian failure occurs when a women’s follicles and eggs are depleted and when the production of estrogen and progesterone ceases. Other factors, including smoking, genetics, exposure to radiation and prior ovarian surgery, may contribute to a more rapid loss of eggs.

Tests can be performed to determine a woman's "ovarian reserve," or the health of the eggs and ovaries, which decreases as a woman ages. The tests determine the basal levels of FSH and estradiol on the third day of a woman's cycle. Although values can vary between laboratories, certain FSH levels are a good indicator of the chance of successful pregnancy due to infertility treatments, including in vitro fertilization (IVF). Woman may also take the clomiphene challenge test, in which women have blood drawn on the third day of their cycle to test for FSH and estradiol, and then take 100 mg of clomiphene citrate for five days (the fifth through ninth days of the cycle) and repeat the test on day 10 of their cycle. If a woman shows a high level of FSH on day 10 compared to the baseline hormone level, this indicates a reduced chance of becoming pregnant.

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