It is thought that a woman is born with all the eggs she will ever have—no new eggs are ever produced. With each menstrual cycle, some of these eggs are used up. There are always fewer and fewer eggs left in the ovary for each subsequent cycle. Furthermore, the healthiest and most responsive eggs are probably the first ones to be ovulated. In other words, when a woman is 20, the eggs being released at that time are very fertile. By the time she reaches 40, the eggs remaining have been through as many as 350 cycles without ovulating—they just aren't as responsive to the stimulation from the pituitary hormones and have less fertility potential. With aging, the functioning of the ovaries declines and the capacity of the remaining eggs to establish a normal pregnancy decreases.
Almost all of the decrease in female fertility with aging is due to this fact. This is known as ovarian reserves. How many functional eggs remain, and determination of ovarian reserves, is often important, particularly in older individuals.
Getting some idea of the extent of an individual's ovarian reserves can be a very important step in providing a couple with some idea about their chances for successful conception, especially before initiating expensive and time-consuming treatments. Both a day-three FSH level and a clomiphene citrate challenge test give us this information. After age 30, and certainly after age 35, these tests should be considered to be sure the ovaries are capable of producing adequate eggs, both in quantity and quality, before initiating some of the more expensive treatment alternatives.