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"We knew from (previous studies) that a uterine Doppler test would predict women at high risk for preeclampsia, (and) our current review shows that aspirin has substantial benefit in these high-risk women," says Aravinthan Coomarasamy, study author and researcher at Birmingham Women's Hospital in Birmingham, England.
A Doppler exam is a type of ultrasound that uses harmless radio waves to measure the speed at which blood flows through the vessels. If the test indicates a blockage or other problem that could hamper blood flow to the uterus, many doctors believe the risk for preeclampsia increases. And it is for these women that experts say the aspirin therapy is promising.
"Very roughly, it halves the risk of preeclampsia in women who have an abnormal uterine Doppler test," says Coomarasamy. Currently, the American College of Obstetrics and Gynecology suggests, up to 8 percent of all pregnant women may be at risk for preeclampsia.
The idea that aspirin may help is not new. In fact, this particular study was a scientific review of more than 300 previous studies that examined the role of aspirin in the prevention of preeclampsia.
Although much of the research yielded conflicting results, the authors believed it was the study designs, not the treatment, that was responsible for the ambiguities.
"Too many women in the (previous analyses) were at very small risks of preeclampsia, (so) basically these women would not have benefited - or benefited only a little - from aspirin," says Coomarasamy.
Including these low-risk women with high-risk women diluted the results, which is why it wasn't clear what aspirin's benefits were for those studies, say the authors.
Once the analysis was limited to only those women at high risk for preeclampsia, and specifically those who were diagnostically proven at risk via the Doppler ultrasound, an aspirin a day was clearly shown to have protective effects.
Other experts respond
"The study was very well done; the authors seem to have performed a thorough search for all relevant studies, and they have evaluated each possibly relevant study for its strengths and weaknesses," says Dr. Fergal Malone, director of the Columbia Perinatal Center at Columbia Presbyterian Medical Center in New York City.
However, he adds that, "One should be careful about making large sweeping policy decisions on the basis of meta-analyses alone - they may help answer a question, but can sometimes show significant results where none truly exist," says Malone. Any flaws in the original studies, he says, will affect the overall results of the new analysis.
The authors went back as far as 1966 to retrieve 332 studies on preeclampsia and aspirin for review. After eliminating all those that were too broadly based or otherwise included too many low-risk or no-risk women, the authors settled on only five studies, which met their rigorous criteria. All were published after 1990, and all included women at very high risk for preeclampsia as determined by a Doppler ultrasound exam.
After analysing the collective data on more than 1 300 women, the authors concluded that when Doppler ultrasound revealed a uterine abnormality, aspirin did "reduce the risk of preeclampsia substantially," says Coomarasamy. The authors say they don't know why or how the aspirin works.
The study was published in the November issue of Obstetrics and Gynecology.
