Consider that at the time a woman is born, her egg supply has been reduced from 6-7 million eggs to 2-3 million. By the time menstruation begins, that number has gone down to 300,000 to 400,000 eggs.
Each month, during a woman’s early years (of menstruating), 1000 eggs will develop, from which the ovary will release only one, the remainder of which will die off. When a woman reaches menopause, usually in her early 50’s, her egg supply has been virtually depleted.
One well done study demonstrated that at age 31, 50% of women begin to experience a decline in their monthly chance of having a child. By age 41, 50% of women will never conceive a child that is genetically theirs. By age 45, very few women retain the ability to conceive using their own eggs.
These statistics, while valid, reflect overall numbers and for each individual, their own personal odds could be better, worse or the same.
A comprehensive Infertility workup will measure ovarian reserve (the number of eggs still present) through bloodwork and ultrasound . This will present a clearer picture of a woman's personal profile.
For more information about the effects of ovarian aging on fertility: