This is one of the first studies to compare the development of preeclampsia to body mass index (BMI - a widely used method of calculating percentage of body fat) as a continuum rather than simply by conventional classification units. It also is the first study to try to determine the mechanisms by which body fat increases preeclampsia risk. Preeclampsia affects about 7 percent of first pregnancies nationwide, and is characterized by high blood pressure and protein in the urine. The condition has negative – sometimes lethal – consequences for the mother and fetus by causing reduced blood flow to organs. Having had preeclampsia also increases maternal risk of cardiovascular disease in later life.
The study was presented by Lisa Bodnar, Ph.D., M.P.H., R.D., a postdoctoral fellow in the lab of James M. Roberts, M.D., director of the Magee-Womens Research Institute, at Experimental Biology 2004, the annual meeting of the American Society for Nutritional Sciences.
The Pittsburgh research team enrolled women at or before the 16th week of their first pregnancy. Preeclampsia usually begins to appear by the 20th week of pregnancy. By the time all women in the study had delivered, 6 percent of the group had developed preeclampsia. After the researchers adjusted their analysis to take into account differences in age, smoking, race, marital status and education, one factor stood out clearly: the higher a woman's body mass index, the more likely she was to develop preeclampsia. This held true for body mass indices of 15 to 35; there were too few women in the study at BMI values over 35 to accurately assess the relationship between BMI and preeclampsia at those values.
Elevated risk of preeclampsia was curvilinear, meaning that the risk of preeclampsia rose sharply as pre-pregnancy BMI increased. Compared to women with a "normal" pre-pregnancy BMI of 21 (130 pounds for a 5-foot, 6-inch woman), women with a pre-pregnancy BMI of 26 (160 pounds), considered "overweight" with a high ratio of body fat to muscle, were twice as likely to develop preeclampsia. Women with a BMI of 30 (185 pounds), considered "obese," were three times as likely to develop preeclampsia as the normal BMI women. Even small increases in BMI were significant.
The most surprising finding, says Dr. Bodnar, was that a woman with a pre-pregnancy BMI of 17, classified as "underweight," was only half as likely to develop preeclampsia as a woman with a BMI of 21, considered "normal weight." This represents the equivalent of a 25-pound difference in weight of two women who are both 5-feet, 6-inches tall.
The finding of increasing risk with elevated BMI, even within each BMI category and even at "normal" BMI compared to lower ones, indicates that fat tissue itself is playing a role in the development of preeclampsia, says Dr. Bodnar. Further analysis of the data suggests that about half of the increased risk of preeclampsia is due to two factors that have long been suspected to play a role in its development: a heightened inflammatory response often associated with high body mass and elevated lipid values in the blood related to being overweight. Dr. Bodnar says understanding the mechanisms of the BMI/preeclampsia relationship could be extremely helpful in finding ways to treat the condition.
While weight loss prior to becoming pregnant may hold the most promise in lowering the risk of preeclampsia, weight loss during pregnancy is discouraged, say the researchers.
The dramatic increase in obesity among Americans is well documented and the epidemic's health consequences on non-pregnant individuals have been widely recognized. Less is known about its impact on reproductive outcomes. The researchers say these results suggest that as obesity continues to rise, so will the rates of preeclampsia.
University of Pittsburgh researchers include Dr. Bodnar and Dr. Roberts, both in the department of obstetrics, gynecology and reproductive sciences, University of Pittsburgh School of Medicine; and Nina Markovic, Ph.D., and Gail Harger, both in the department of epidemiology, University of Pittsburgh Graduate School of Public Health.
Additional Contact Information: Sarah Goodwin, Experimental Biology 2004
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