NIH News Release
National Institute of Child Health
and Human Development

Wednesday, March 11, 1998
5:00 PM Eastern Time
Robert Bock
(301) 496-5133

Study Finds Aspirin Does Not
Prevent High-Risk Cases of Preeclampsia

Contrary to prevailing medical opinion, low dose aspirin therapy during pregnancy does not prevent preeclampsia in women who are at high risk for the disease, according to the largest, most comprehensive study of its kind.

Preeclampsia is a disorder of pregnancy that can strike without warning, causing high blood pressure and protein in the urine. In turn, preeclampsia may progress to eclampsia--hypertension and generalized convulsions--which may be fatal.

The trial, reported in the March 12 New England Journal of Medicine, was conducted by researchers at the National Institute of Child Health and Human Development (NICHD) Network of Maternal-Fetal Medicine Units, a network of 13 clinical research centers studying various aspects of maternal-fetal medicine.

"Women at high risk of preeclampsia should not be given aspirin therapy to prevent the condition, as our study shows it doesn't work." said Dr. Donald McNellis, the NICHD project officer for the study and an author of the paper. "Although our study did not find any ill effects from the aspirin, this drug does have known potential side effects."

Side effects of aspirin include irritation of the stomach and intestine and spontaneous hemorrhaging. During pregnancy, aspirin may increase the risk for abruptio placentae--separation of the placenta from the uterine wall at birth.

The authors note that preeclampsia is a leading cause of maternal death. About 5 percent of first-time mothers and 1 to 2 percent of mothers having subsequent pregnancies develop the condition. Even in cases where the condition does not progress to eclampsia, the children born to mothers with preeclampsia may be small for their gestational age or may be born prematurely; this may, in turn, place them at risk for a variety of other complication.

Although the high blood pressure accompanying preeclampsia can be treated with blood pressure lowering drugs, the only curative treatment for the overall condition is immediate delivery.

Dr. McNellis added that previous large studies failed to find benefit for the aspirin therapy and have been criticized as not testing women who were actually at high risk. He added that several smaller trials found that low doses of aspirin could reduce complications of preeclampsia. In response to these early studies, health care professionals began prescribing aspirin to prevent the condition.

Dr. McNellis noted, however, that the discrepancy between the current finding and the earlier studies could be explained. Many of the earlier studies, he said, included both women at high risk for the condition as well as those at low risk. The reduced incidence of preeclampsia in the low-risk women could have resulted in the risk of preeclampsia appearing smaller in the group as a whole. Another possibility is "publication bias," which occurs because studies that have positive findings are likely to get published in the medical journals while those with negative findings are not.

In all, the researchers studied 2539 women at high risk of preeclampsia. The women were between their 13th and 26th week of pregnancy. Of these, 1254 were given 60 mg. of aspirin a day while 1249 received a placebo. For the aspirin group, the incidence of preeclampsia was 18 percent and for the placebo group, 20 percent--a difference that was not statistically significant.

Dr. McNellis noted that the study tested the aspirin therapy in the four categories of women comprising the total number of women at high risk of preeclampsia: those who have diabetes before pregnancy, those with high blood pressure, those pregnant with more than one child, and those who had experienced preeclampsia in an earlier pregnancy. For all four categories, the difference of preeclampsia in both the aspirin and placebo groups was not statistically significant. Similarly, the study also showed that the aspirin treatment had no effect on the complications of preeclampsia: the incidence of preterm birth, being small for gestational age, or dying during birth.

"We conclude that aspirin should not be given to prevent preeclampsia in women with pregestational insulin-treated diabetes, chronic hypertension, multifetal gestation, or preeclampsia in a previous pregnancy," the authors wrote.