On the new finding, lead researcher Rebecca Moore of the University of Colorado Health Sciences Center in Denver, said, "The reason for this increased risk is unclear, but may possibly be associated with maternal poverty and social deprivation."
Using birth certificate data on infants born in Colorado from 2000 to 2006, the researchers analyzed risk factors for pre-eclampsia and PIH in more than 362,000 mothers. All of the women were free of chronic medical conditions at delivery. Women with pre-eclampsia develop rapid increases in blood pressure, along with kidney damage. Pre-eclampsia is a very common condition and the third most common cause of maternal death in the United States. The cause is unknown and there is no known cure, although the problem usually resolves after delivery.
The overall rate of PIH/pre-eclampsia was 3.3 %. The study confirmed the known risk factors, including age over 35 years, first pregnancy, multiple gestation (twins or more), and gaining more than 30 pounds during pregnancy. Smoking was associated with a lower risk of pre-eclampsia (but smoking increases the risk of other pregnancy complications). After adjusting for all of these factors, the researchers identified some intriguing new risk factors for PIH/pre-eclampsia. Women living in rural counties were at increased risk: 56 percent higher than for women in other areas.
There was also a link to education, with a 19 % increase in risk for women who had some college education (compared to a high school education). Although the rate of PIH/pre-eclampsia was also higher for women with more than a college education, the difference was not significant. "These novel risks were independent of other risk factors, including adequacy of prenatal care," Dr. Moore adds.
The new information linking PIH/pre-eclampsia risk to the mother's social characteristics is an important first step to toward identifying new, non-traditional risk factors. "Although traditional risk factors for pre-eclampsia and PIH are well recognized, these diseases remain enigmatic, and there is no known effective way to reduce their incidence," says Dr. Moore. "Nontraditional risk factors may be of great importance in the design of future interventions to prevent the occurrence of PIH and pre-eclampsia, but data regarding these risk factors are scarce."
The data provided in the study were all self-reported, and are therefore subject to bias. Also, the study database did not include information on some potentially important factors, including the women's body weight and income level.