A meta-analysis 2 included 45 trials with a total of 20 909 pregnant randomized to 50-150 mg of aspirin daily. When aspirin was initiated at≤16 weeks, there was a significant reduction and a dose-response effect for the prevention of pre-eclampsia (RR 0.57, 95% CI 0.43 to 0.75; P < .001), severe pre-eclampsia (RR 0.47, 95% CI 0.26 to 0.83; P = .009), and fetal growth restriction (RR 0.56, 95% CI 0.44 to 0.70; P < .001) with higher dosages of aspirin being associated with greater reduction of the 3 outcomes. Similar results were observed after the exclusion of studies at high risk of biases. When aspirin was initiated at >16 weeks, there was a smaller reduction of pre-eclampsia. Aspirin initiated at >16 weeks was not associated with a risk reduction or a dose-response effect for severe pre-eclampsia (RR 0.85, 95% CI 0.64 to 1.14; P = .28) and fetal growth restriction (RR 0.95, 95% CI 0.86-1.05; P = .34).
A Cochrane review [Abstract] 1 included 77 studies with a total of 40 249 women. Antiplatelet agent (mostly low-dose aspirin - 50 to150 mg) was compared with placebo/no antiplatelet agent. Antiplatelet agents reduced the risk of proteinuric pre-eclampsia by 18%, number needed to treat for one women to benefit (NNTB) 61 (95% CI 45 to 92). There was a small (9%) reduction in preterm birth <37 weeks, NNTB 61 (95% CI 42 to 114) and a 14% reduction infetal deaths, neonatal deaths or death before hospital discharge, NNTB 197 (95% CI 115 to 681). Antiplatelet agents probably slightly increase postpartum haemorrhage > 500 mL.
| Outcome | Relative effect(95% CI) | Assumed risk- Placebo/no antiplatelet | Corresponding risk - Antiplatelet agents | No of Participants(studies)Quality of the evidence |
|---|---|---|---|---|
| Proteinuric pre-eclampsia | RR 0.82(0.77 to 0.88) | 92 per 1000 | 16 fewer per 1000(22 fewer to 11 fewer) | 36 716(60) High |
| Any reported infant death (fetal, neonatal, or before hospital discharge) | RR 0.85(0.76 to 0.95) | 33 per 1000 | 5 fewer per 1000(9 fewer to 1 fewer) | 35 391(52) High |
| Preterm birth(before 37 weeks' gestation) | RR 0.91(0.87 to 0.95) | 175 per 1000 | 16 fewer per 1000(23 fewer to 9 fewer) | 35 212(47) High |
| Small-for-gestational age | RR 0.84(0.76 to 0.92) | 47 per 1000 | 7 fewer per 1000(11 fewer to 3 fewer) | 35 761(50) High |
| Pregnancy with serious adverse outcome (composite including maternal death, baby death, pre-eclampsia, small-for-gestational age, preterm birth) | RR 0.90(0.85 to 0.96) | 197 per 1000 | 20 fewer per 1000(30 fewer to 8 fewer) | 17 382(13) High |
| Postpartum haemorrhage > 500 mL | RR 1.06(1.00 to 1.12) | 143 per 1000 | 9 more per 1000(0 fewer to 19 more) | 23 769(19) Moderate |
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