Overweight and Pregnant: Why BMI Matters

What extra pounds mean to you and your baby.

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Overweight women who want to get pregnant soon might want to reconsider their timing: A higher BMI is associated with an increased risk of fetal death, stillbirth, and infant death, according to a JAMA review examining 38 studies on the topic. Researchers suggest women take these findings into consideration, if they're planning to conceive.

"Doctors have long known that very obese women risk pregnancy complications, but research indicates that even women who are not hugely overweight have elevated risks," says Hugh M. Ehrenberg, M.D., of Case Western Reserve School of Medicine in Cleveland, the author of a different study on obesity and pregnancy, published in the American Journal of Obstetrics and Gynecology. The list of potential complications is formidable: hypertension, preeclampsia and eclampsia, gestational diabetes (which can lead to overly large babies), C-sections and postoperative complications.

There's more: A study from the Centers for Disease Control and Prevention confirmed that a woman who is overweight before becoming pregnant is two to three times more likely to have a baby with heart abnormalities, spina bifida or other birth defects. Another study links excess weight and obesity to lower levels of the lactation hormone prolactin after childbirth, which may explain why overweight women tend to stop nursing earlier than average-weight women.

To prevent such problems, a woman should, if possible, be at or close to her ideal weight when she becomes pregnant. Sometimes losing just 5 to 10 percent before getting pregnant is enough to decrease her risk factors. But since not every pregnancy is planned, many overweight women want to know whether—and how—they can safely deal with their weight while pregnant. The thinking on this issue is changing.

Revising the Weight-Gain Rules

"We used to tell all women to gain 25 to 35 pounds during pregnancy, but now we suggest that an overweight or obese woman gain only 15 to 25 pounds," says Paula Bernstein, M.D., an OB-GYN in Los Angeles and a co-author of Carrying a Little Extra: A Guide to Healthy Pregnancy for the Plus-Size Woman (Penguin Putnam, 2003). That's because an overweight woman already has a greater store of the nutrients her developing baby needs.

A body mass index (or BMI, a formula that relates weight to height) of 25 to 29.9 is considered overweight, while a BMI greater than 30 is the current standard for obesity. "Even a woman with a BMI of 25 begins to be at risk for pregnancy complications," Bernstein says. (Speaking of BMI, check out our BMI Calculator, here.)

Though doctors currently advise against losing weight during pregnancy, Ehrenberg is investigating whether doing so might benefit a very heavy woman's health without risking her baby's. One concern is that ketosis—the incomplete metabolism of fatty acids that occurs when carbohydrates are severely limited and weight loss occurs rapidly, as is the case with high-protein diets—may harm a fetus. But a balanced, calorie-restricted diet can result in weight loss without causing ketosis.

"Until now, the research regarding weight loss during pregnancy has been done on women who are already at an ideal weight," Ehrenberg says. "But as a significant number of women now are overweight when they become pregnant, we need to establish whether they can lose weight through diet and exercise without harming their baby."

Safe Ways to Gain Less

If you're overweight and pregnant, you can safely limit your weight gain by working with a registered dietitian and a certified trainer, each of whom have expertise in pregnancy nutrition and prenatal exercise. Bernstein advises significantly obese women to have a thorough cardiovascular checkup before starting an exercise program that can be followed during pregnancy and beyond.

"Whatever you do, do it in moderation," Bernstein says. "This isn't the time to train for a marathon or join a high-impact aerobics class. Start any new exercise slowly and build up gradually. Avoid exercise that can damage your joints, which are more vulnerable during pregnancy. Walking and swimming are excellent choices."

Pregnancy can be an ideal time to tackle weight issues—for your sake and your child's as well. Your habits will influence his, so think of pregnancy as an opportunity to become a model of healthy eating and exercise.

Related: 33 Reasons to Exercise Now