To investigate the effect of the timing of chemoprophylaxis on venous thromboembolisms (VTEs) and bleeding rates in patients undergoing major abdominal surgery. Background: Postoperative bleeding and VTE incur significant morbidity, mortality, and health care costs. Chemoprophylaxis is used routinely to prevent VTEs but increases bleeding risk. The perioperative timing of chemoprophylaxis initiation may influence both VTE and bleeding risks. The optimal window for commencing chemoprophylaxis in the perioperative period is unclear. Methods: MEDLINE, EMBASE, Cochrane Library, and Web of Science databases were searched using PRISMA guidelines. Randomized trials and cohort studies published between January 1, 2000 to May 10, 2022, which reported on chemoprophylaxis timing as well as the incidence of VTE and bleeding after elective abdominal surgery were meta-analyzed. Results: From 6175 studies, 14 (24,922 patients) were meta-analyzed. Bariatric (4 studies), antireflux (1 study), hepato-pancreatic-biliary (5 studies), colorectal (1 study), ventral hernia (1 study), and major intraabdominal surgeries (2 studies) were included. Chemoprophylaxis was initiated before skin closure in 10,403 patients, and postoperatively in 14,519 patients. Both symptomatic [risk ratios (RR), 0.81; 95% CI, 0.45-1.43; P = 0.460] and overall (RR, 0.74; 95% CI, 0.45-1.24; P = 0.250) VTE rates were comparable between study groups. Compared with postoperative chemoprophylaxis, early usage increased the risk of all bleeding (