By Allison Van Dusen

updated 11:49 a.m. ET April 15, 2007

Having kids is one of those things most of us take for granted — until the day rolls around when we decide we want children and experience trouble conceiving.

But while some causes of infertility are hereditary or due to health problems beyond your control, reproductive experts say there are a few steps you can take to preserve and enhance your ability to have children.

A healthy lifestyle alone won't necessarily protect you from infertility, cautions Dr. Steven Ory, of IVF Florida Reproductive Associates and an associate clinical professor at the University of Miami.

But it may help.

"The benefits of exercise, a proper diet, maintaining good general health are very important," Ory says. "People who are very unhealthy have a much higher incidence of infertility."

Studying the stats
The prevalence of infertility in the United States today may be as high as one in six couples of childbearing age.

Dr. Samuel Pang, medical director of the Reproductive Science Center of New England, estimates that about one-third of his patients' difficulties is due to a woman's health problem and another third is due to a problem with a man's sperm.

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The other third consists of couples whose age is affecting their ability to conceive, a group that's growing as more people delay having children until later in life. Doctors say maternal age in particular is one of the most important factors couples need to consider.

"Sociologically we've changed, but our biology hasn't changed very much," says Dr. Richard Scott, director of Reproductive Medicine Associates of New Jersey. "Women make a lifetime supply of eggs when they're in the fetus. When they're gone, they're gone."

In fact, a woman's fertility starts to measurably decline around age 27, due to the depletion and aging of her eggs. For those under 30, it's estimated that the chance of getting pregnant in any one cycle is 20 percent to 30 percent. By age 40, it falls to 5 percent, according to the American Fertility Association.

Guys, listen up
But it's not just women who need to pay attention to the ticking of the clock, says Dr. Harry Fisch, director of the Male Reproductive Center at New York-Presbyterian Hospital/Columbia University Medical Center and author of "The Male Biological Clock."

Men over 35 are twice as likely to be infertile as those under 25. Studies also are showing that, as with older women, older men are more likely to have children with birth defects due to the decreased genetic quality of their sperm.

"Every cell in the body ages," Fisch says. "Why would you think the sperm or testicles don't age?"

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