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From birth, females have a fixed—though plentiful—supply of eggs (ovarian reserve). As a woman ages past her mid-30s, her eggs gradually degrade, making it less likely that she will naturally conceive, or that an assisted reproductive technology (ART) procedure will result in pregnancy and a healthy baby.

Among American women in their 20s to mid-30s, over 35 out of 100 give birth for each ART cycle using their own eggs. As women age, the live ART birth rate gradually drops: ¹

• To about 20 out of 100 for each IVF cycle by age 39.
• To 5 or less out of 100 for each IVF cycle in women over age 43. Many women over age 40 choose to use donor eggs, which greatly improves their chances of giving birth to a healthy child.

While there is no definitive test of ovarian reserve, a woman's follicle-stimulating hormone (FSH) level can be measured to evaluate how well her ovaries are working. A high FSH level is a sign that the body is trying to stimulate the ovaries to make more egg follicles, but the ovaries are not responding and conception is unlikely.

A woman's FSH level can be tested using a blood sample:

• Early in her menstrual period, around day 3.
• Before and after a clomiphene challenge test, early in her menstrual period.