A number of studies over the years have found an association between obesity and pregnancy complications, including pre-eclampsia (hypertension), gestational diabetes and stillbirth, but there was little evidence of a direct, cause-and-effect relationship. A new, large-scale study from researchers at the Harvard School of Public Health (HSPH) and the Karolinska Institutet in Stockholm, Sweden, found that an increase in body mass index (BMI) between first and second pregnancies was associated with adverse outcomes, findings that support a possible causal relationship between obesity and pregnancy complications. The results are published in the September 30 edition of The Lancet.
Obesity is a growing problem worldwide. The World Health Organization considers it a major global health threat. In Sweden, during the years of the study (1992-2001), the percentage of pregnant women either overweight or obese increased from 25 percent to 36 percent. In the U.S., the prevalence of obesity in women aged 20-39 years jumped from 9 percent in 1960-1962 to 28 percent in 1999-2000.
The researchers, Eduardo Villamor, assistant professor of international nutrition at HSPH and Sven Cnattingius, professor of epidemiology at Karolinska Institutet, looked at a study population of more than 150,000 Swedish women having their first and second births between 1992 and 2001. They calculated BMI at the first prenatal visit of each pregnancy (BMI is weight in kilograms divided by the square of height in meters). The researchers then calculated the difference between BMI at the beginning of the first and second pregnancies. The adverse outcomes that Villamor and Cnattingius looked at during the second pregnancy included maternal complications, such as pre-eclampsia, gestational diabetes, gestational hypertension and caesarean delivery, and perinatal complications, such as stillbirth and large-for-gestational-age birth.
The results showed that weight gain between first and second pregnancies was associated with an increased risk of all these overweight and obesity-related adverse outcomes. 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 to 40 percent. A gain of three or more BMI units showed a 63 percent greater chance of stillbirth compared to a gain of less than one BMI unit and also a greater effect on all other complications. Additionally, the researchers found that the risk of adverse outcomes increased even in women who were not overweight, but who gained a modest amount of weight between pregnancies. For example, if a woman who was 5 ft., 5 in., tall and weighed 139 lbs. (giving her a BMI of 23, not considered overweight) gained 6.6 lbs. (1 BMI unit) between her first and second pregnancies, her average risk of gestational diabetes would increase by more than 30 percent. If she gained 12.2 pounds (2 BMI units), her risk would increase 100 percent. The risk would continue to climb if she gained more weight and became obese.
"Previous studies had looked at the risk of pregnancy complications in relation to a single snapshot measure of pre-pregnant weight, and found that obesity appeared to increase the frequency of such complications," said Villamor. "In this study, we went further back in time to look at how women arrived at that pre-pregnant weight. 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 illnesses. In addition, weight loss in overweight women appeared to lower their risk of pregnancy complications."
One limitation of the study was that it was not possible to differentiate whether weight gain between pregnancies could have been due to lack of weight loss after the first pregnancy, excessive weight gain during that initial pregnancy or a weight increase between the time after the first delivery and the next conception. There was also the possibility that other factors or illnesses could be associated with both weight gain and adverse pregnancy outcomes. In a commentary that accompanies the article, Aaron Caughey, Assistant Professor of Obstetrics, Gynecology, and Reproductive Sciences at the University of California, San Francisco, notes that breastfeeding could be one such factor, since women who breastfeed retain less weight post partum while breastfeeding for longer periods of time lowers diabetes risk. But overall, the authors believe the results provide strong evidence that weight gain before pregnancy could lead to a number of complications of pregnancy.
"A key public health message from our study is that women of normal weight should avoid gaining weight between pregnancies. In addition, overweight and obese women are likely to benefit from weight loss if they are planning to become pregnant. Further research is needed to find the most effective interventions that prevent interpregnancy weight gain and postpartum weight retention," said Villamor.
The authors clarified that the conclusions of this particular study apply to weight gain before pregnancy only, not to weight gain during pregnancy.
Contact: Todd Datz
Harvard School of Public Health