The New Pregnancy Math

Ready for some new math? Researchers in the UK have devised a mathematical formula that predicts a woman's odds of conception in any given month based on her age and the length of time she's been trying to get pregnant. It's an interesting way to wager when you'll get that plus sign you've been hoping for, but there's a little more to it than that. Researchers say that understanding pregnancy odds could help couples determine how long to wait before seeking help.

How does it work? The analysis, developed at the University of Warwick and the London School of Economics, uses the age of the woman and the number of menstrual cycles over which the couple has been trying for a baby to determine a probability of conception within the next month. Statistically, the longer a couple tries to have a baby without any luck, the lower the chances they will conceive—and the higher the chances they might need to consult with a fertility specialist.

For example, a woman who is 25 years old has an 18 percent chance of conceiving within the first three months of trying. At six months, her chances are 15 percent. At nine months, odds stand at 12 percent. At one year, the odds of pregnancy drop to 10 percent.

However, when researchers looked at women in other age groups, they found out that, for older women, the odds fell to 10 percent much sooner. For a 35-year-old, the odds for conceiving within the first three months hover around 12 percent, but by six months of trying, the odds are at 9 percent. For a woman who is 40, the chance of getting pregnant within the first three months is already at 7 percent.

Taking all this into account, researchers say that instead of using the general "one-year rule" when it comes to fertility intervention, maybe it's the number—10 percent—that should be the new benchmark for determining who's at risk for fertility problems.

So, how do you feel about your odds? Mom Jennifer Ferruzza of Seattle, Washington, says that if she could have bet on her pregnancy, she would have been the long shot.

"I was 39 and got pregnant the first month we tried. If only I could have gotten the spread on that one!"