Adherence to aspirin in the prevention of pregnancy complications: a systematic review

The risk-reducing effect of aspirin on pregnancy complications is suggested to depend on therapy adherence. To gain more insight, we reviewed the available literature on aspirin adherence in pregnancy. Three parameters were investigated: methods of assessment of aspirin adherence, definitions of aspirin adherence, and association between aspirin adherence and pregnancy complications. RCTs investigating the preventive effect of aspirin on pregnancy complications including aspirin adherence were included. An update search of the Cochrane review of Duley 2019 was performed, two trial registries were searched and forward citation chasing was performed on May 31, 2025. Risk of bias was assessed according to the criteria of Higgins, checking for selection, performance, detection, attrition, reporting and other bias. Data were summarized and tabulated, resulting in descriptive analyses. Due to the heterogeneity of the data, no meta-analysis could be performed. Adherence of aspirin use during pregnancy was tested with clearly described method(s) and results in 16/63 RCTs on the use of aspirin in pregnancy. From the applied methods, pill count was most frequently performed (n = 14), followed by interview (n = 9) and thromboxane B2 (TxB2) measurements (n = 2). Most studies used a multi-measure approach. Definitions of good adherence were provided in 11 studies: percentage tablet intake (n = 10) with thresholds between ≥50 % to ≥90 %, and/or drop of ≥50 % in serum TxB2 (n = 2). Most common pill count threshold ≥80 % (n = 6) showed mean adherence of 79.5 %. An association between good adherence and lower incidence of PE was found in 2 studies. Limitations of the systematic review were the lack of a general definition of aspirin adherence and the large heterogeneity in methods, therefore being unable to perform a meta-analysis or draw hard conclusions. In conclusion, 25 % of the RCTs on aspirin in pregnancy investigated aspirin adherence including clearly described method(s) and results and in only 11 a definition on aspirin adherence was set. Multi-measure approach is recommended as test method. Future studies should focus on threshold of good aspirin adherence in pregnancy and dose–response effect.