Objectives: To determine the effect of extended 28-day venous thromboembolism (VTE) prophylaxis on VTE rates within 30 days and 6 months of debulking surgery for epithelial ovarian cancer (EOC).Methods: This historical cohort study estimated incidence of VTE within 30 days and 6 months of debulking surgery in women who received extended VTE prophylaxis (28 days low-molecular-weight heparin [LMWH]) versus those who did not. Women undergoing primary, secondary, and interval debulking for EOC from January 1, 2009, to June 30, 2014 were included. Chronic anticoagulation and VTE before surgery or hospital discharge were exclusion criteria. Secondary analysis was performed on a subset of women who underwent primary debulking surgery (PDS).Results: Of 590 women undergoing debulking surgery, 90 (15.3%) received prophylactic LMWH. Within 30 days, 16 women were diagnosed with VTE, for a cumulative incidence of 2.9% (95% CI 1.5–4.2), which was not significantly different between those who did and did not receive LMWH (P = .32; 30-day rate, 1.2% vs 3.1%). By 6 months, 53 women were diagnosed with VTE, for a cumulative incidence of 9.9% (95% CI 7.3–12.4) with no significant difference between those who did and did not receive LMWH (P = .98; 6-month rate, 10.2% vs 9.9%).The PDS subset included 484 women; 63 (13.0%) received extended LMWH. Within 30 days of surgery, 15 women were diagnosed with VTE, for a cumulative incidence of 3.3% (95% CI 1.6–4.9), with no significant difference between those who did and did not receive LMWH (P = .47; 30-day rate, 1.6% vs 3.4%). One patient died of a PE within 30 days of surgery; she had PDS and did not receive LMWH. By 6 months, 41 women were diagnosed with VTE, for a cumulative incidence of 9.3% (95% CI=6.6–12.0) and no significant difference with or without LMWH (P = .54; 6-month rate, 6.8% vs 9.6%).Although postoperative LMWH was not associated with VTE rates, history of VTE and measurable residual disease were associated with increased risk of VTE at 30 days and 6 months. High surgical complexity was associated with increased 6-month VTE risk (Table 1).Table 1Univariable analysis of factors affecting venous thromboembolism rates within 30 days and 6 months of all debulking surgeries and primary debulking surgeries. Objectives: To determine the effect of extended 28-day venous thromboembolism (VTE) prophylaxis on VTE rates within 30 days and 6 months of debulking surgery for epithelial ovarian cancer (EOC). Methods: This historical cohort study estimated incidence of VTE within 30 days and 6 months of debulking surgery in women who received extended VTE prophylaxis (28 days low-molecular-weight heparin [LMWH]) versus those who did not. Women undergoing primary, secondary, and interval debulking for EOC from January 1, 2009, to June 30, 2014 were included. Chronic anticoagulation and VTE before surgery or hospital discharge were exclusion criteria. Secondary analysis was performed on a subset of women who underwent primary debulking surgery (PDS). Results: Of 590 women undergoing debulking surgery, 90 (15.3%) received prophylactic LMWH. Within 30 days, 16 women were diagnosed with VTE, for a cumulative incidence of 2.9% (95% CI 1.5–4.2), which was not significantly different between those who did and did not receive LMWH (P = .32; 30-day rate, 1.2% vs 3.1%). By 6 months, 53 women were diagnosed with VTE, for a cumulative incidence of 9.9% (95% CI 7.3–12.4) with no significant difference between those who did and did not receive LMWH (P = .98; 6-month rate, 10.2% vs 9.9%). The PDS subset included 484 women; 63 (13.0%) received extended LMWH. Within 30 days of surgery, 15 women were diagnosed with VTE, for a cumulative incidence of 3.3% (95% CI 1.6–4.9), with no significant difference between those who did and did not receive LMWH (P = .47; 30-day rate, 1.6% vs 3.4%). One patient died of a PE within 30 days of surgery; she had PDS and did not receive LMWH. By 6 months, 41 women were diagnosed with VTE, for a cumulative incidence of 9.3% (95% CI=6.6–12.0) and no significant difference with or without LMWH (P = .54; 6-month rate, 6.8% vs 9.6%). Although postoperative LMWH was not associated with VTE rates, history of VTE and measurable residual disease were associated with increased risk of VTE at 30 days and 6 months. High surgical complexity was associated with increased 6-month VTE risk (Table 1).