Low-dose aspirin use in pregnancy and the risk of preterm birth: a Swedish register-based cohort study

Preterm birth is the leading cause of neonatal mortality and morbidity. Women who have had a previous preterm birth are at increased risk for preterm birth in their subsequent pregnancies. Low-dose aspirin use reduces the risk for preterm birth among women at risk of developing preeclampsia, however, it is unclear whether low-dose aspirin may reduce the risk of recurrent preterm birth.This study aimed to investigate the association between low-dose aspirin use and preterm birth among women with a previous preterm birth.We conducted a Swedish register-based cohort study and included women who had a first and second pregnancy between 2006 and 2019, with the first pregnancy ending in preterm birth (medically indicated or with spontaneous onset <37 weeks of gestation). The association between low-dose aspirin use and preterm birth in the second pregnancy was estimated via logistic regression via standardization and expressed as marginal relative risks with the 95% confidence interval.Among the study cohort (N=22,127), 3057 women (14%) were prescribed low-dose aspirin during their second pregnancy and 3703 women (17%) gave birth prematurely. Low-dose aspirin use was associated with a reduced risk for preterm birth, (marginal relative risk, 0.87; 95% confidence interval, 0.77-0.99). There were no statistically significant associations between low-dose aspirin use and an altered risk for moderate preterm birth, defined as birth between 32 and 36 weeks' gestation (marginal relative risk, 0.90; 95% confidence interval, 0.78-1.03), or very preterm birth, defined as birth <32 weeks' gestation (marginal relative risk, 0.75; 95% confidence interval, 0.54-1.04). Regarding the onset of preterm birth, low-dose aspirin use was associated with a reduced risk for spontaneous preterm birth (marginal relative risk, 0.70; 95% confidence interval, 0.57-0.86) but no reduction in the risk for medically indicated preterm birth (marginal relative risk, 1.09; 95% confidence interval, 0.91-1.30) was observed.Among women with a previous preterm birth, low-dose aspirin use was associated with a reduced risk for preterm birth. When investigating preterm birth by onset in the second pregnancy, low-dose aspirin use was associated with a reduced risk for spontaneous preterm birth. Our results suggest that low-dose aspirin may be an effective prophylaxis for recurrent preterm birth.