So-called “super obese” women are at sharply higher risk of a potentially fatal pregnancy complication compared to their normal-weight peers, especially if they gain a lot of weight while they’re pregnant, new research shows.

About five percent of pregnant women develop high blood pressure and kidney problems, known as pre-eclampsia, usually after the 20th week of pregnancy, according to the National Heart, Lung and Blood Institute. The condition carries serious risks for both mother and baby.

While a high pre-pregnancy body mass index (BMI, a measure of weight in relation to height used to gauge whether a person is at a healthy weight) is a known risk factor for pre-eclampsia, there is little information on risks for the growing number of super-obese women. These are women with a BMI of at least 50, for example a woman 5 feet 5 inches tall weighing 300 pounds or more.

The number of super-obese people in the US has quintupled over the past two decades, Dr. Hamisu Salihu of the University of South Florida in Tampa and colleagues note in the obstetrics and gynecology journal, BJOG.

To examine how pre-eclampsia risk might be influenced by obesity type, the researchers looked at birth records from Missouri from 1989 to 2005, including more than 850,000 single live births. More than three-quarters of the women were at a normal weight before getting pregnant, while 21 percent were obese. Less than a half percent, or 3,001 women in all, were super-obese.

The overall risk of pre-eclampsia was 4.5 percent; it was about 3 percent for normal weight women, 9 percent for obese women, and 13 percent for super-obese women.

Risk of pre-eclampsia was tripled for the obese women (BMI of 30 or above) compared to the normal-weight women (BMI of 25 or less). The risk for the super-obese women was nearly fivefold greater.

The faster a woman gained weight during pregnancy, the higher her risk of pre-eclampsia, no matter what her BMI. Super-obese women with high weight gain (over 0.7 kilograms, or about 1.5 pounds, per week) were 13 times as likely as normal-weight women with moderate weight gain (from 0.5 to 1.5 pounds a week) to develop pre-eclampsia.

“Obesity, especially super obesity, needs to be a focus” of preconception care, Salihu told Reuters Health, “so that women who are obese reduce their weight to an appropriate level to become pregnant.”

Super-obese women who do get pregnant can be monitored carefully to reduce their risk of complications, he added, for example by ensuring that they gain an appropriate amount of weight. And after a woman has her baby, Salihu said, doctors should work with her to help her lose weight and make subsequent pregnancies less risky for both mother and baby.

SOURCE:  BJOG, online May 19, 2010.

By Anne Harding
NEW YORK (Reuters Health)