Three decades ago, maternal-fetal medicine specialist Yvonne Thornton, M.D., was determinedly shedding the 67 pounds she gained during her first pregnancy when she found out she was expecting another baby. Thornton vowed not to let that derail her healthy eating habits again. “There was a strong dictum back then that no matter what you weighed, you should gain 26 to 35 pounds during pregnancy or risk fetal death,” says Thornton, now an OB-GYN professor at New York Medical College. “But we put our diabetic pregnant patients on a sensible diet that was safe, so I figured it was OK for me.”
Thornton focused on eating healthfully and watching portion sizes, recording everything she ate in a food journal. Her weight stayed steady at 222 pounds until she delivered a healthy 8-pound, 8-ounce girl at 43 weeks. “I felt a hundred times better than I did during my first pregnancy,” she says. Two months later, she was down to 160 pounds.
Buoyed by her own success, Thornton conducted a randomized clinical trial over several years to show that emphasizing nutrition rather than weight would lead to healthier pregnancies for obese women (those with a prepregnancy body-mass index, or BMI, of 30 or higher). Her study of 232 such women found that those who followed a doctor-monitored nutrition plan gained less weight, had fewer C-sections, were less likely to develop gestational diabetes and retained less weight after delivery. Fifty-seven women gained fewer than 10 pounds, and 23 of them lost weight during pregnancy with no negative consequences.
When weight may be misleading Doctors monitor a mother’s weight gain as a way to evaluate fetal growth, but regularly measuring fundal height, the distance from the top of the mom’s uterus to her pubic bone, is a better indicator, Thornton maintains. When the Institute of Medicine updated its pregnancy weight-gain guidelines in 2009, the authors said there was not enough data to support recommending anything less than an 11- to 20-pound gain for obese women. However, in a study of nearly 74,000 women presented at the Society for Maternal-Fetal Medicine’s annual meeting this past February, very obese women (a BMI of 35 or higher) who gained fewer than 11 pounds suffered no ill consequences—and neither did their babies. Those with a BMI of 30 to 34.9 who gained fewer than 11 pounds did have a greater risk of delivering a low-birth-weight baby.
Thornton offers these suggestions for very heavy women: 1) Do not try to limit weight gain without your doctor’s supervision; 2) Weigh in only at prenatal visits, and consider asking not to be told the results; 3) Eat twice as well, not twice as much (but don’t go under 2,000 high-quality calories a day); 4) Faithfully keep a food journal (be honest!) and discuss it at prenatal visits.