Nine RCTs (n=1,347, range 26 to 560) were included in the review. Two trials recruited women at 16 weeks or less (n=229), one between 17 and 19 weeks (n=104) and six 20 weeks or more (n=1,014). Five studies conducted double-blinding and six conducted intention-to-treat analysis. Loss to follow-up was less than 5.7%. There was no evidence of publication bias. There was a discrepancy in patient numbers between the tables and the text.
A greater reduction in the incidence of pre-eclampsia was associated with ASA treatment beginning earlier in gestation compared with treatment beginning in late gestation. A significant 52% reduction in the risk of pre-eclampsia compared with the control group was observed when treatment commenced at 16 or less weeks gestation (RR 0.48, 95% CI 0.33 to 0.68, NNT=5; two studies). At both 17 to 19 weeks (RR 0.55, 95% CI 0.17 to 1.76; one study) and 20 or more weeks gestation (RR 0.82, 95% CI 0.62 to 1.09; six studies) there was a non-significant reduction in the risk of pre-eclampsia compared with the control group.
Significant reductions in the incidence of severe pre-eclampsia (RR 0.10, 95% CI 0.01 to 0.74), gestational hypertension (RR 0.31, 95% CI 0.13 to 0.78) and IUGR (RR 0.51, 95% CI 0.28 to 0.92) were associated with commencing ASA treatment at 16 or less weeks gestation.
There was no significant heterogeneity for these comparisons.