While previous studies had noted an association between weight gain and pregnancy complications, this new research is the best evidence thus far for a cause-and-effect relationship, the researchers said.
"We are contributing evidence to strengthen the argument for causality," said Dr. Eduardo Villamor, assistant professor of international nutrition at the Harvard School of Public Health and lead author of the study, which was published in the Sept. 30 issue of The Lancet. "As far as we could humanly show, the evidence is pretty robust," he said.
The practical implications are clear. "Being of normal weight and trying to lose weight between pregnancies would be very helpful," added Dr. Ellen Landsberger, assistant professor of obstetrics and gynecology and women's health at Albert Einstein College of Medicine in New York City, who also has a degree in nutrition.
Paradoxically, another new study found that women who had regularly exercised for four or more hours a week in the previous nine years or less were 40 percent less likely to have a live birth after their first cycle of in-vitro fertilization, compared to women who didn't exercise.
The study also found that regular exercisers were twice as likely to have an implantation failure or pregnancy loss after undergoing in-vitro fertilization (IVF). Cardiovascular exercise, in particular, was associated with a 30 percent lower likelihood of successful live birth compared with women who didn't exercise. It was unclear why this was the case, although hormones certainly could be involved, the authors theorized.
This study, by researchers at Brigham and Women's Hospital in Boston, was published in the October issue of the journal Obstetrics & Gynecology.
Overweight and obesity have become one of the world's greatest public-health challenges. In the United States in 1999-2000, 28 percent of women aged 20 to 39 were obese, up from only 9 percent in 1960-62.
And some relationship, whether causal or not, between overweight and obesity and pregnancy complications has long been suspected.
For the new study, investigators from Harvard and Sweden took advantage of Sweden's excellent health records, studying more than 150,000 Swedish women with first and second births between 1992 and 2001.
Body mass index (BMI, a ratio of weight to height) was calculated at the first prenatal visit of each pregnancy. The difference in BMI was then correlated with complications during second pregnancies, including preeclampsia (hypertension), gestational diabetes, gestational hypertension and Caesarean delivery as well as stillbirth and large-for-gestational-age birth.
A gain of one to two BMI units increased the risk of gestational diabetes, gestational hypertension or large-for-gestational age birth an average of 20 percent to 40 percent. A gain of three or more BMI units resulted in a 63 percent greater chance of stillbirth compared to a gain of less than one BMI unit. It also had a greater effect on all other complications, the researchers found.
Even modest amounts of weight gain in normal-sized women caused problems. If a 5-foot-5-inch woman weighting 139 pounds (a BMI of 23, considered normal) gained 6.6 pounds -- or one BMI unit -- between the two pregnancies, her average risk of gestational diabetes increased more than 30 percent. A gain of 12.2 pounds (two BMI units) increased the risk 100 percent.
"One of the key findings of our study is that we found that weight gain between pregnancies increased the risk of these complications even in women who had never been obese or overweight," Villamor said.
"It appears it is not necessary for women to become overweight or obese, a few kilos might carry a risk," he continued. "One message could be that women should not gain any weight. They should try to go back to their pre-pregnant weight and should not gain any weight after the first pregnancy. Obviously, overweight and obese women should try to reduce their weight and that's likely accompanied by other health benefits."
More information
To calculate your BMI, visit the National Heart, Lung, and Blood Institute.
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