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Efficacy of obstetric thromboprophylaxis and long‐term risk of recurrence of venous thromboembolism

Objective. To study the efficacy of thromboprophylaxis with low molecular weight heparin (LMWH) in pregnant women with one previous venous thromboembolic event (VTE). Secondary aims were to study the long‐term risk of secondary recurrence, bleeding and obstetric complications. Design. A prospective national study of long‐term LMWH thromboprophylaxis in Sweden. Settings. All hospitals in Sweden during January 1998–December 2002, Participants. Pregnant women with one previous VTE and controls drawn from the Swedish Medical Birth Registry. The women were cross‐matched with the Swedish Hospital Discharge Register to identify all recurrences and to ascertain the annual risk of recurrence. Main Outcome Measures. Recurrence of VTE, bleeding complications at delivery and obstetric complications. Results. 326 of 393 registered women could be evaluated. The relative risk reduction in VTE was 88%. There was an absolute increased risk of VTE during the thromboprophylaxis period: 1.2% compared to 0.2% among controls (p<0.001). The risk during the immediate post‐treatment period (43–100 days post‐partum) was increased 28‐fold. The annual incidence of VTE after delivery was 1%. The risk of hematoma and major blood loss at delivery was increased during thromboprophylaxis (p<0.001). There were no differences in the incidences of preeclampsia, intrauterine growth restriction or placental abruption. Conclusions. The relative risk reduction in VTE during thromboprophylaxis was 88%. After pregnancy, the annual long‐term risk of recurrence was 1%. The risk was most pronounced in the post‐treatment period. There was an increased risk of bleeding complications among women given LMWH, but there was no effect on obstetric complications.