Many women assume that as long as they are still getting their periods, they have plenty of healthy eggs left. But this is not quite true. There may be some eggs, but they tend to be of such poor quality that they stop dividing soon after fertilization or implantation in the uterus.
Although menopause -- the permanent cessation of periods -- happens, on average, at age 51.5, fertility begins to decline around age 35. By 38, this decline accelerates and by 40, ''it nosedives," said Dr. David Keefe, medical director of reproductive medicine at Tufts-New England Medical Center.
In addition to age, other predictors of declining fertility include a family history of menopause before 35, smoking, surgery to remove an ovary or a cyst on an ovary, and having had chemotherapy or radiation for cancer, said Dr. Howard Zacur, director of the division of reproductive medicine and infertility at Johns Hopkins.
There is a blood test for a hormone called FSH, which stands for follicle stimulating hormone, that can, to some degree, predict a woman's fertility. FSH, made in the pituitary gland, tells eggs in the ovaries to mature. As a woman ages and her ovaries begin to run out of eggs, FSH levels rise.
Fertility clinics use FSH tests (usually done on day two, three or four of a woman's cycle) to tell whether she is likely to get pregnant with in vitro fertilization. In general, if the FSH level is more than 10 International Units per milliliter, a woman is unlikely to get pregnant.
Although FSH levels can fluctuate from month to month, in general, ''you are only as fertile as your worst FSH score," Keefe said. It's like a gas gauge on a car, he said. Once the needle registers empty, even if that's only when you're turning a corner, you are close to having an empty tank.
In the future, better tests may be available that directly measure a hormone secreted by a developing egg. Until then, Keefe recommends reading a book called ''What Every Woman Should Know About Fertility and Her Biological Clock" by Cara Birrittieri.