Special Report: Calculating Your Clock

Cara Birrittieri of Medfield had no problems having her first child at forty. So she was shocked when a year later she tried to have a second and found out it was too late.

"I was devastated," Birrittieri said.

Many women assume they can wait until later in life to get pregnant.

"You see these magazine covers with celebrities holding their twins at age 45 or older," Birrittieri said.

Problem is, figuring out when your clock will slow down isn't an exact science.

"There's a huge variability from woman to woman," Dr. David Keefe of Tufts New England Medical Center said.

Doctors can do ultrasounds to determine how many eggs a woman has left. But that won't tell them if they're any good.

"They'll tell you a lot more about the number than the quality or the chance they'll become a baby," Dr. Keefe said.

To help women better calculate their own clocks, Birrittieri wrote a book in which she offers a simple test based on scientific studies to help women get a general idea of how many years of fertility she might have left.

First, ask your mother when she hit menopause. Then subtract ten years from that.

"About half the variability in fertility is genetic," Dr. Keefe said.

Add two years if mom was a smoker.

Then subtract two years if you smoke yourself.

"Typically women who smoke have half the fertility rate of women who don't smoke," Dr. Keefe said.

Subtract six months if you've been exposed to second hand smoke and another two to three years if your mother smoked while pregnant.

"It's kind of hard to get that info cause a lot of women won't admit that they smoked when they were pregnant with you," Birrittieri said.

Subtract six months for any exposure to environmental toxins, such as arsenic, mercury, or lead.

Then add a year if you live a healthy lifestyle by eating well and exercising.

After calculating her own clock, 22-year-old Meghan Flattery of Millis realized she could lose her fertility at only 31.

"I was shocked that that early my fertility could decline," Flattery said.

She says she's glad she found out, before it was too late.

"I probably would have just waited and not thought twice about it," Flattery said.

As for Birrittieri, she ended up having to find an egg donor for her baby. And now she wants other women to know that if they don't calculate their clock it could end up being ‘too late.’

While this test can give you an idea of what your fertility might be, it should not be considered an exact science.

For more information on calculating your clock you can go to Birrittieri’s website listed below.

(Copyright 2005 by The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)