It's a statistic that concerns many women 35 and older. But Jean Twenge, author of the article "How Long Can You Wait to Have a Baby?" in The Atlantic says women in this age group don't need to panic.
"That period between 35 and 40 is more fertile than many people believe," she said.
Twenge, who had her third child at 40, says much of the research women fear the most is based on old data.
"Modern studies suggest that the statistics weren't as scary for older women," said Twenge.
Twenge says a new study found that among 38-and 39-year-olds, 80 percent of them got pregnant naturally within six months. Much older research says one in three women ages 35 to 39 will not get pregnant after a year of trying.
"Everybody is different, but by and large earlier is better," said Dr. Mark Surrey, director of the Southern California Reproductive Center.
He says whether a couple can conceive depends on many variables.
"Does the egg and the sperm meet, do they fertilize, do they divide normally, are they genetically normal?" he said.
Despite all the variables, Dr. Jamie Grifo says the real decline in fertility is for women over 40.
"Every two years, after age 40, fertility is cut in half again. I don't want to be alarmist, but I also don't want to make it sound like, 'Hey! You should count on being able to get pregnant,' because you shouldn't count on being able to get pregnant," he said.
Still, Twenge says women in their 30s should feel empowered, not panicked.
"There's no real reason to be scared having a baby after 35. Yes, the risks go up, but the number of women who have these problems, thankfully, is still very low, even in a woman's late 30s," she said.
Most experts agree the fertility rate of women over 35 is about 10 percent each month, and at age 40 it drops to about 5 percent even with in vitro fertilization.