News Release

Nighttime is the right time to prevent preterm labor with aspirin

Peer-Reviewed Publication

American Heart Association

ORLANDO, Sept. 26 – Low-dose aspirin prevented preterm labor in pregnant women at risk for high blood pressure, but only if they started therapy before the 16th week of pregnancy, and only if they took it at night, according to research presented today at the American Heart Association's 56th Annual High Blood Pressure Research Conference.

"This is the first study to show that aspirin at bedtime works and aspirin at awakening does not," says lead author Ramon C. Hermida, Ph.D., director of the bioengineering and chronobiology laboratories and a professor at the University of Vigo in Vigo, Spain. He added that earlier studies may have had conflicting results because they used lower aspirin doses, started therapy later in pregnancy or gave it at different times of the day.

The double-blind, randomized controlled trial followed 341 pregnant women who were at high risk for blood pressure problems due to obesity, family or personal history of gestational high blood pressure or preeclampsia, or a history of miscarriage.

The women were divided into six groups and given 100 mg of aspirin or placebo at one of three times: on awakening, eight hours later or at bedtime. Treatment started at 12 to 16 weeks of pregnancy.

Compared with placebo, there was no effect on blood pressure when aspirin was taken in the morning. However, blood pressure was significantly reduced when the women took aspirin eight hours after awakening and, to a greater extent, when they took it at night. When aspirin was taken at bedtime, the differences between the aspirin and placebo groups were significant: The incidence of preeclampsia was 14.3 percent in the placebo group, but just 1.7 percent in the aspirin group. Gestational hypertension occurred in 30.4 percent of women in the placebo group, but only 6.8 percent in the aspirin group. Also, 17.9 percent of women in the placebo group had pre-term labor, while none of the women taking aspirin delivered early.

"All the results are really surprising," Hermida says. "This study shows remarkable reductions in all gestational high blood pressure and preeclampsia complications. But there is more: Women who took aspirin at bedtime gave birth to children 250 grams (about 9 ounces) heavier on average compared to women who took placebo or who took aspirin in the morning instead of at night."

Hermida stresses that women should not take aspirin during pregnancy without a doctor's supervision. Although the low dose used in this study appears safe, higher doses such as those used in pain relief do carry a risk of increased bleeding at delivery. "Despite potential advantages, not necessarily all pregnant women could benefit from taking aspirin," he says.


Diana Ayala, M.D., Ph.D., was co-author of the report.

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