Experts say the main reason for the drop is that women are born with all the eggs they will ever have, and the supply of eggs goes down with age. "The quality of the eggs also goes down, which increases the likelihood of miscarriage in older women," says Adelina Emmi, M.D., associate professor of reproductive endocrinology and infertility at the Medical College of Georgia. "You may hear about celebrities having twins at 50, but you don't always know the details, like whether donor eggs were used."

And though men produce sperm most of their lives and don't experience the sudden drop in fertility that women do, a man's fertility may decrease gradually over time. "As men age, their fertility declines later and less dramatically than it does in women," Emmi says. "There is also evidence that the risk of gene defects in sperm goes up with age."

The risk of infertility also goes up when either partner has had diseases or surgery that could damage the reproductive organs. For example, a major complication of sexually transmitted diseases for women is pelvic inflammatory disease (PID). This infection can lead to infertility because it causes scarring in the uterus and fallopian tubes. Men may have reproductive abnormalities due to prostate surgery or a disorder of the testes resulting in abnormal sperm production.

"This is an area of medicine where getting a good patient history really counts," Emmi says. "We can pick up all kinds of things that people may not realize affect their fertility--from chronic conditions like prediabetic states and thyroid disorder, which can interfere with ovulation, to blood pressure medication, which can lower a man's sperm count."

Lifestyle risk factors that can impair fertility in men and women include smoking, alcohol and drug use, and sexually transmitted diseases. In women, being overweight or underweight can interfere with the production of estrogen, a female hormone that regulates the menstrual cycle and ovulation. According to the ASRM, too much body fat causes a woman to produce too much estrogen. Too little body fat causes a woman to produce too little estrogen.

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