Watch Your Weight

5. Watch your weight. The average woman should put on 25 to 35 pounds during pregnancy. Gain too much and you up your odds of complications like gestational diabetes and preeclampsia, which increase preterm labor risk. Talk to your doctor if you were heavy before pregnancy -- it's recommended that overweight and obese women gain less weight (usually 15 to 25 pounds) -- and follow a nutritious diet and exercise to lower your risk.

Underweight women should be concerned too. Research has shown that women who have a body mass index (BMI) under 20 (a healthy BMI is 20 to 25) are less likely to carry their baby to term. These women may be prone to nutritional deficiencies that adversely affect the fetal environment, says Robert Goldenberg, M.D., professor of obstetrics and gynecology at the University of Alabama School of Medicine, in Birmingham.

To calculate your BMI, divide your weight before pregnancy in pounds by your height in inches. Divide that number by your height in inches again, then multiply by 703. Women are considered overweight if their BMI is over 25, and obese if it's over 30. If you're under- or overweight, talk to your doctor about seeing a nutritionist for help with proper diet and weight gain.

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