Sarah Joyce was in her physical and reproductive prime, a competitive long-distance runner in her 20s, but when she tried to get pregnant, her body failed her.
At the height of her training, the petite athlete from Indianapolis ran 60 miles a week and weighed in at 85 pounds, with not an ounce of body fat.
Like other female athletes, the physical demands she placed on her body over a lifetime of sports and training for two marathons had taken its toll. By 2009, she had dropped 10 pounds off her 5-foot, 1-inch frame.
When Joyce arrived at Genesis Fertility and Reproduction in New York City, where she eventually conceived, she was diagnosed with hypothalamic amenorrhea. Her periods had stopped because her body was no longer making the estrogen to produce eggs.
As women are increasingly involved in competitive sports or rigorous recreational activities, being too fit can hurt pregnancy chances, according to fertility specialists.
Now, at 30, and with a newborn daughter, Joyce's hindsight is 20-20.
"I may have been too intense and I think if I try again to have another baby, I will change my regimen," she said.
According to Dr. David Hoffman, a IVF Florida Reproductive Associates, female athletes are more likely to have ovulation and menstruation disorders that thwart conception.
"A lot of them are runners and ballet dancers because they starve themselves, and gymnasts with no body fat -- anyone who takes anything to the extreme," he said.
Hoffman knows firsthand. He treated nine-time swimming medalist and Olympic swimmer Dara Torres when she was trying to conceive. She made a dramatic comeback at the age of 41 at the 2008 Games in Beijing.
Torres became a hero to millions of American women when she spoke out about the trouble she had conceiving her now 4-year-old daughter, Tessa Grace, the daughter she shares with Hoffman. [Hoffman did not treat Torres for her successful pregnancy.]
Just this week, " target="external">tennis player Gigi Fernandez admitted publicly that she had seven failed fertility treatments before conceiving with her partner, and doctors blamed it on her athletic career.
The winner of 17 Grand Slam doubles titles before she retired number one at the age of 33 in 1997, Fernandez was told her eggs were told old.
"It was crushing," Fernandez told the New York Times. "I felt almost like I wished I would have never played tennis.
"As an athlete, you have this attitude, 'I can do anything with my body,'" she said. "That's how you think. So your biological clock is ticking, but you're in denial."
Fernandez finally became pregnant in 2008 using donated eggs and sperm, giving birth to twins after her 45th birthday.
About 1 in 6 women or 7.3 million couples have difficulty conceiving a child, according to the " target="external">American Fertility Association (AFA).
Reproductive specialists say about 12 percent of those infertile women are athletes, and the problem is becoming more acute now that more women are testing the limits of their body -- and their biological clock.
Fertility Problems Come From Over-Exercise
"Most of the problems we see are low body fat or over-exercising and getting infrequent periods or they lose them altogether," said Hoffman. "In the brain, the hypothalamus just goes ice cold. It's like a stress reaction -- fight or flight."
The syndrome is nothing new. As early as 1993, the American College of Sports Medicine warned about the reproductive fallout of excessive exercise in women.
It noted that recreational jogging -- only 12 to 18 miles a week -- can result in poor follicular development, decreased estrogen and progesterone secretion and absent ovulation.
Reproductive specialists say they see amenorrhea most frequently in ballet dancers and long-distance runners. Some studies have shown 44 percent of all ballet dancers have no periods.
But it's also a problem in college athletes.
Katlyn Haycock, a 20-year-old exercise science major and former Syracuse University Women's tennis team player, has experienced amenorrhea and low estrogen levels.
"It started happening in high school," she said. "I was working out too much and my body didn't have enough energy to maintain it."
"Freshman year I never took days off except for Christmas and Easter," said Haycock. "I started overtraining definitely during my freshman year, and it continued in my sophomore year. And then I started to burn out."
She said she has never worried about her lack of periods or her fertility.
"It's a little more convenient with playing a sport, you didn't have to worry about it," she said. "It honestly didn't bother me too much."
Estrogen is an essential protective hormone for women and long-term data show an impact on cardiovascular health, according to Dr. Wendy Chang, a reproductive endocrinologist at the Southern California Reproductive Center in Beverly Hills.
"I think the bigger issue, more than the health consequences now, are that women who have exercise-induced amenorrhea have decreased bone density," said Chang. "And even though they are belly-dancers and doing weight-bearing exercises, they are more prone to stress fractures."
But the biggest factor in infertility is age, when female athletes delay childbearing for their careers.
Women Delay Childbearing Past 35
Many women delay having children, and as a result, 20 percent of all women in the U.S. have their first child after 35, according to the RESOLVE, the National Infertility Association.
"We peak at 25," said Chang. "A woman in her early 20s has a 30 percent chance of getting pregnant. That drops to 12 to 13 percent at 35 and by 40, it's 5 percent."
Some specialists, including Chang, recommend that women who intend to devote their childbearing years to professional sports consider freezing their eggs when they are at their peak in their 20s.
"Pregnancy rates five years ago were in the 3 percent range," said Chang. "Nowadays vitrification techniques have significantly improved egg survival and pregnancy rates and studies suggest that if you freeze eggs under age 35, pregnancy rates improve."
"The best quality eggs are in 20-year-olds," she said. "They should consider it as long as they have a balanced and detailed discussion with a reproductive endocrinologist about the risks and benefits. Armed with good information, it's a good option for women."
As for Sarah Joyce, she hopes to soon resume running with her newborn Makayla in her new running stroller.
But she is more cautious about her health than ever before.
"There should be a balance between eating healthy and continuing to exercise," she said. "I have made a conscious effort to add more to my diet. Now, my husband says stop putting nuts and cheese on everything."
Joyce now sees a nutritionist in anticipation of a second pregnancy, and hopes to have "the best of both worlds."
"I keep up my activity and I eat healthy," she said. "I can't wait to get back running. I have a few high goals and the rest is for fun."
ABC News' On Campus reporter Marlei Martinez contributed to this story.