Incidence of pulmonary and venous thromboembolism in pregnancies after in vitro fertilization with fresh respectively frozen‐thawed embryo transfer: Nationwide cohort study

Background The assisted reproductive technique in vitro fertilization (IVF) is associated with an increased risk of venous thromboembolism (VTE) and pulmonary embolism (PE) during the first trimester. Objectives To compare the incidence of VTE and PE during the first trimester of IVF pregnancies using fresh or frozen‐thawed embryo transfer to that during natural pregnancies. Patient/Methods Nationwide Swedish registry‐based cohort study of women who gave birth (n = 902 891) at the age of 15‐50 years to their first child from the 1st of January 1992 until the 31st of December 2012. Exposure groups were IVF with fresh respectively frozen‐thawed embryo transfer. Incidences of VTE and PE were calculated, and time‐varying hazard ratios estimated for all trimesters after fresh respectively frozen‐thawed embryo transfer IVF and compared to natural conception. Results and Conclusion Women giving birth after fresh embryo transfer IVF had a more than eightfold increased incidence of venous thromboembolism (hazard ratio [HR] 8.96, 95% CI 6.33 to 12.67) and pulmonary embolism during the first trimester, (HR 8.69, 95% CI 3.83 to 19.71) compared to women giving birth after natural conception. The incidence of VTE in women giving birth after frozen‐thawed embryo transfer was not increased during the first trimester. To conclude, fresh embryo transfer IVF was associated with a significantly increased incidence of VTE and PE during the first trimester. These results suggest that frozen‐thawed embryo transfer could be a preferred method of IVF with a minimised maternal risk.