The large study offers some of the best evidence yet confirming a long suspected, direct link between maternal overweight and obesity and pregnancy complications such as pre-eclampsia, gestational diabetes, and stillbirth, its authors say.
"Both obesity and these pregnancy complications could have similar causes, so we have not known if it was actually the weight that was really responsible for the pregnancy risk," researcher Eduardo Villamor, MD, of the Harvard School of Public Health in Boston, tells WebMD.
But, "it turns out that women do not need to become overweight or obese in order to increase their chances of having poor gestational outcomes; only a relatively modest increase in weight between pregnancies could lead to serious illness," Villamor says. Villamor is an assistant professor of international nutrition at Harvard.
The study is published in the Sept. 30 issue of The Lancet.
Even Normal-Weight Women at Risk
Along with study co-author Sven Cnattingius, MD, of Stockholm's Karolinska Institute, Villamor tracked weight changes among more than 150,000 Swedish women between their first and second pregnancies. The average time between the birth of the first child and estimated date of conception of a second child was two years.
First, they looked at the mother's body mass index (BMI) at the first prenatal visit for each pregnancy. (BMI compares height to weight and is used to evaluate if a person is of normal weight, overweight, or obese.)
Then they looked at the women's pregnancy complications.
Added Pounds, More Problems
They found that extra weight not lost after a first pregnancy, or gained before a second, increased the risk for many bad outcomes in pregnancy and delivery.
A weight gain of 17 pounds by a normal-weight 5-foot-2-inch woman, for example, was associated with a 63% increased risk of delivering a stillborn baby, compared with a similar-height woman who gained just a few pounds.
Even modest increases in weight among normal-weight women increased the risk of problems during a second pregnancy.
Villamor and Cnattingius calculated that gaining just 6.6 pounds between pregnancies could increase a normal-weight, 5-foot-5-inch woman's risk of developing gestational diabetes by more than 30%. If the same woman gained 12 pounds, her risk could increase by 100%.
Helping New Moms Get Fit
Understanding how weight gain affects pregnancy is particularly important in light of the dramatic rise in overweight and obesity among women of childbearing age in developed countries such as the U.S. and Sweden.
Between 1960 and 2000, the incidence of obesity among women in the U.S. between the ages of 20 and 39 tripled, from 9% to 28%.
And during the period in which the study was conducted, between 1992 and 2001, the percentage of pregnant women in Sweden who were overweight or obese increased from 25% to 36%.
In an editorial accompanying the study, Aaron Caughey, MD, an assistant professor of obstetrics, gynecology, and reproductive sciences at the University of California, San Francisco, called for more research to find ways to help women lose postpartum weight as well as extra weight put on between pregnancies.
"With the recent sustained increase in obesity, particularly in the developed world, such interventions are likely to affect not only future pregnancy outcomes but also long-term outcomes in women's health," Caughey wrote.