Incidence, clinical characteristics, and timing of objectively diagnosed venous thromboembolism during pregnancy1

OBJECTIVE: To estimate the incidence, timing, and associated clinical characteristics of objectively diagnosed pregnancy-associated venous thromboembolism. METHODS: We retrospectively reviewed venous thromboembolism cases (deep venous thrombosis and pulmonary embolism) that occurred between 1978 and 1996. Study inclusion criteria required the objective diagnosis with either Doppler ultrasound, venography, impedance plethysmography, pulmonary angiography, ventilation-perfusion scanning, or computed tomography or magnetic resonance imaging. RESULTS: Among 268,525 deliveries there were 165 (0.06%) episodes of venous thromboembolism (one per 1627 births). There were 127 cases of deep venous thrombosis and 38 cases of pulmonary embolism. Only 14% (23 of 165 patients) had a history of venous thromboembolism. Most cases of deep venous thrombosis were in the left leg (104 of 127, 81.9%), with nearly three quarters of them (94 of 127, 74.8%) occurring during the antepartum period. Among cases of antepartum deep venous thrombosis, half were detected before 15 weeks' gestation (47 of 95, 49.5%), and only 28 cases occurred after 20 weeks (P < .001). Most of the pulmonary embolisms occurred in the postpartum period (23 of 38, 60.5%) and were strongly associated with cesarean delivery (19 of 36,470 compared with four of 232,032, P < .001). CONCLUSION: The incidence of venous thromboembolism during pregnancy is lower than has been previously described. Most cases occurred in the antepartum period, with the risk of deep venous thrombosis appearing to begin even before the second trimester.