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A meta-analysis of 31 randomised trials reports that taking low dose aspirin during pregnancy reduces the risk of pre-eclampsia by 10% compared with taking placebo (relative risk 0.90, 95% CI 0.84 to 0.97).0.97). The authors found a similar reduction in the risk of preterm birth (0.90, 0.83 to 0.98) but not in risk of perinatal death or the infant being small for gestational age. Aspirin reduced the risk of any serious adverse outcome by 10% overall (0.90, 0.85 to 0.96). The authors estimate that 51 women would need treatment to prevent one serious adverse outcome. Aspirin seemed safe in the short term for both mother and baby.
The trials included more than 32000 women, most of whom had a low or moderate risk of pre-eclampsia. Despite these numbers, the authors could not pinpoint any subgroup most likely to benefit. Even those with risk factors such as diabetes or hypertension were no more likely to benefit than other women.
So who should doctors treat? A linked editorial (doi: 10.1016/S0140-6736(07)60713-2) says aspirin is justified only for pregnant women who are almost certain to get pre-eclampsia, including those who have had it at least twice before. The judgment is much harder for women at lower risk. We simply don't know enough about aspirin's long term effects on the unborn child.