ObjectiveTo systematically review the safety of low molecular weight heparin (LMWH) in pregnancy. MethodsPubMed, EMbase, The Cochrane Library, WanFang Data, VIP, and CNKI databases were electronically searched to collect randomized controlled trials (RCTs) and cohort studies on the safety of LMWH in pregnancy from inception to March 30th, 2020. Two reviewers independently screened literature, extracted data, and assessed the risk of bias of included studies. Meta-analysis was then performed using RevMan 5.3 software. ResultsA total of 77 RCTs and 13 cohort studies were included. The results of meta-analysis showed that LMWH increased the incidence of postpartum hemorrhage (RR=1.50, 95%CI 1.00 to 2.25, P=0.05). However, there was no significant difference. The incidence of hematological adverse events was different from the results of RCTs and cohort studies. The results of RCT subgroup analysis showed that LMWH increased ecchymosis at the injection site (RR=1.60, 95%CI 1.24 to 2.08, P=0.000 4). However, the incidence of overall skin system adverse events did not increase significantly. LMWH reduced the incidence of cardiovascular adverse events (RR=0.18, 95%CI 0.07 to 0.46, P=0.000 3). LMWH failed to increase the occurrence of fetal congenital malformations, digestive system, central nervous system, skeletal system, and systemic adverse events. ConclusionsCurrent evidence suggests that LMWH is relatively safe to use during pregnancy. However, whether it increases postpartum hemorrhage and hematological adverse events is unclear. Due to limited quality and quantity of the included studies, more high-quality studies are required to verify the above conclusions.