Low risk of venous thromboembolism following early pregnancy loss in pregnancies conceived by IVF

Abstract STUDY QUESTION What is the risk of venous thromboembolism (VTE) in the 12 weeks following early pregnancy loss in pregnancies conceived by IVF? SUMMARY ANSWER The VTE risk is low in the 12 weeks following early pregnancy loss in pregnancies conceived by IVF. WHAT IS KNOWN ALREADY There is an excess VTE risk during first trimester in complete IVF pregnancies leading to delivery. It is unknown whether this excess VTE risk also is present in IVF pregnancies terminated by early pregnancy loss (implantation failure, missed abortion, or spontaneous or induced abortion before 10 completed gestational weeks). STUDY DESIGN, SIZE, DURATION A nationwide registry-based cohort study including all Danish IVF pregnancies registered in the Danish IVF Register between 1995 and 2005. Women who underwent frozen embryo replacement or oocyte donation were not included. PARTICIPANTS/MATERIALS, SETTING, METHODS We included all 24 931 IVF treatments leading to pregnancy among 19 260 women. We identified 16 701 complete IVF pregnancies, 7567 IVF pregnancies with early pregnancy loss, and 663 IVF pregnancies terminated by late abortion (≥gestational weeks 10). We followed women for 12 weeks after termination of pregnancy and calculated the absolute risk of VTE during follow-up with 95% CI. As a relative risk estimate, we calculated the risk ratio for VTE following IVF pregnancies with early loss compared to the VTE risk following complete IVF pregnancies. MAIN RESULTS AND THE ROLE OF CHANCE During the 12 weeks of follow-up, only one case of VTE occurred in the group of early pregnancy loss, none in the late abortion group, and 13 VTE cases occurred in complete IVF pregnancies. The VTE risk per 10 000 pregnancies was 1.3 [0.03-7.4] for IVF pregnancies with early loss and 7.8 [4.1-13.3] for complete pregnancies; the corresponding risk ratio was 0.17 [0.02-1.3]. Thus, we found a low absolute VTE risk in the 12 weeks following early pregnancy loss in IVF pregnancies. The relative VTE risk was low in comparison to the VTE risk in early gestation reported for complete IVF pregnancies and for Danish background pregnancies. LIMITATIONS, REASONS FOR CAUTION Despite the use of complete nationwide data, only a few VTE events were available for the statistical analyses thereby limiting the precision of our estimates. We included both inpatient and outpatient VTE hospital diagnoses, but we cannot rule out the occurrence of VTE cases not diagnosed at hospital and hence not registered in