Angie and Kelly Malchose weren't even trying to get pregnant when they conceived their son, Jordan. So five years later, when the Malchoses, of Dickinson, North Dakota, decided to go for baby number two, they expected instant success. But to their dismay, months passed and there was still no baby.

Finally, Angie, 29, checked in with her obstetrician, who diagnosed secondary infertility -- the inability to get pregnant after having given birth one or more times. "When he told me this was a common problem, I was shocked. I didn't think infertility applied to people who'd already had kids." In fact, nearly one-quarter of couples seeking treatment are already parents. And as the Malchoses discovered, the struggle can take a devastating toll on moms and dads.

Unraveling a mystery

Doctors consider a couple infertile when they can't conceive after a year of unprotected sex (though most physicians include women who suffer repeat miscarriages in the definition). But many experts say people should seek medical advice after 6 months -- not 12 -- especially if the woman is over 35. "As you near forty, you don't have time to waste -- and you should see your ob-gyn or a specialist sooner rather than later," says Leon Speroff, M.D., professor of obstetrics and gynecology at the Oregon Health & Science University, in Portland. Unfortunately, parents may be slower to seek help than childless couples, figuring if it happened once, it'll happen again. And ob-gyns may take a wait-and-see approach with patients who already have a child.

While there are no hard-and-fast figures, most cases of secondary infertility stem from the female half of the equation. The most common glitch? Aging eggs. Women are born with a finite number of eggs, which can suffer chromosomal damage over time. A defective egg is less likely to be fertilized or, if it is, to result in a viable embryo. This explains why a woman's fertility starts to dip noticeably when she hits 30, taking a plunge after 35. A 27-year-old has a 75 percent chance of conception after 12 months of unprotected sex; over age 35, the chance drops to 40 percent.

Vicki and Pat Daly of New York City believe their plan to add to their family was curtailed by Vicki's biological clock. She was 39 when she had Julia, who's 8. "Three years later, we tried again, but nothing happened," says Pat. "When the infertility doctor didn't find anything overtly wrong, we assumed that we'd waited too long. It was so upsetting." (Because sperm production never stops, men don't experience a dropoff in fertility until their 50s or 60s -- and even then it's minimal.)

Passing time also means that a mom may have developed a medical condition she didn't have before. An underactive thyroid, for example, can trigger infertility by nudging the body's hormonal balance out of kilter. Endometriosis, a disease in which tissue that normally lines the uterus migrates into the abdomen, can prevent conception by causing adhesions on the ovaries or Fallopian tubes that may make them function improperly. Women in their 30s and 40s are also prone to fibroids, benign growths in the uterus that can prevent an embryo from latching onto the lining of the womb.

Men's fertility can also be compromised by such conditions as prostate infections, sexually transmitted diseases, and diabetes, or even alcohol abuse. Jeanne Ryan of Shaker Heights, Ohio, stopped using birth control after her younger child turned 2, expecting to conceive within a couple of months, as she had with her son and daughter. When she and her husband finally went to the doctor a year and a half later, they discovered that he had varicoceles (enlarged veins in the testes), which were thwarting sperm production. "We're still hoping that a good sperm might leak through, but basically, we feel blessed to have two children and don't think we'll pursue aggressive treatment," she says.

Occasionally, parents can't conceive again because they've been marginally fertile all along and were simply lucky before. And some couples fall into the frustrating category of "unexplained infertility." Fortunately, this is a rare diagnosis among those with kids. "If a couple was able to get pregnant before, in most cases something specific must have changed," says A.F. Haney, M.D., professor of obstetrics and gynecology at the University of Chicago School of Medicine.

Mary Garner Ganske, a mom of two, writes frequently for Parenting. She lives in Ohio.