Pregnancy

For most women, pregnancy is a nod to the divine. On the other hand, it's also a matter of mathematics -- from calculating due dates to weighing the risk of birth defects. To help you better understand where doctors get the figures they throw your way during those 9 months (actually, 40 weeks), here's a guide to some of the magic numbers of pregnancy.

40

The number of weeks after the first day of the last menstrual period (LMP). Most obstetricians use this time span to determine an expectant woman's due date, since a typical pregnancy lasts about 38 weeks from conception, and conception usually takes place about two weeks after the LMP. But this formula isn't foolproof. If you don't have a regular 28-day cycle, you may not ovulate exactly two weeks after your LMP, says Carol Wood, associate professor of nursing at the University of Maine.

35

The age at which a woman's risk of delivering a baby with a chromosomal disorder, such as Down syndrome, increases to the point that experts may offer amniocentesis, genetic screening, or some other test. (It's at the age of 35 that the risk of a genetic disorder -- about 1 birth in every 192 -- becomes higher than the risk of miscarriage from amniocentesis.) After age 35, the chance of miscarriage complications, such as hypertension or gestational diabetes, and the need for a cesarean delivery may also rise.

25 to 35

The recommended number of pounds to gain during pregnancy -- if your weight is normal for your height to begin with. Expectant women who are very overweight may put on as few as 15 pounds; those who are very thin may gain slightly more. Experts agree that women should focus mostly on proper diet and nutrition.

72

The maximum number of hours after a woman's membranes rupture that doctors will allow before inducing labor. Though some doctors induce after 12 hours, more and more physicians opt for between 48 and 72 hours. This is a change from years ago, when a woman would be induced within 24 hours of her water breaking to reduce the risk of infection. Today, if a woman hasn't given birth within 18 hours of her water breaking, she's given antibiotics until delivery is complete.

1 to 2

The number of weeks past the due date when many obstetricians choose to induce labor. While you may feel ready to get on with it when you're a week overdue, worrying about complications or feeling uncomfortable generally aren't good enough reasons to bring on labor artificially, says Wood. About 10 percent of women carry beyond 42 weeks -- and 95 percent of their babies are born perfectly healthy.