ABC News

Many women worry they're waiting too long to get pregnant, but a doctor and professor of psychology says many women are misguided on their fertility window.

Dr. Jean Twenge, a professor at San Diego State University and the author of the recent article "How Long Can You Wait to Have a Baby" in The Atlantic magazine, is asking, what's the rush?

"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," she said.

Twenge says a new study found that among 38 and 39 year olds, 80 percent became pregnant naturally within six months. Older research has indicated that 1 in 3 woman ages 35 to 39 will not get pregnant after a year of trying.

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Rose Pondell, 33, is a lawyer for Family Formation Law Center, which helps couples who want children, and is eight months pregnant. She disagrees with Twenge's article, and doesn't see how waiting could be good.

"You hear a lot in the media that once you hit 35, you really are out of time," Pondell said. "I'm having my first child at 33. Ideally I'd like to have two or, maybe, three children. Who knows how long that will take?"

Dr. Jamie Grifo with the New York University Fertility Center says the real decline in fertility is for women over 40.

"Every two years, after age 40, fertility is cut in half again," Grifo said. "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."

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," Twenge said. "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 30's."