Association of Pulmonary and Venous Thromboembolism in IVF Pregnancies After Fresh and Frozen Embryo Transfer: a Population-based Cohort Study

A population-based cohort study with data prospectively collected in the nationwide registries in Sweden. Through the national registries the investigators collect data on all women with a child born in Sweden between 1992-2012 and these will be linked to data from the registries by the use of the unique personal identity number.

Study population All women who gave birth at the age of 15-50 years to their first child from the 1st of January 1992 until the 31st of December 2012 comprise the study population. Women will be categorized as either giving birth after IVF or natural conception pregnancies. Data on whether IVF was performed by the use of fresh or frozen embryo transfer will be retrieved.

The registries The national registries in Sweden are considered to have good coverage and validity in general and the Swedish Medical Birth Register (MBR) by the National Board of Health and Welfare in Sweden includes annually 97-99.5 % of all child births in Sweden, up until 2005 all live child births and stillborn from week 28 and thereafter including all child births from week 22. From the MBR and the Swedish national quality registry of assisted reproductive technology, the Q-IVF, the investigators collect information on pre-pregnancy and pregnancy variables. From the Patient registry (PR) ICD codes for PE and overall VTE will be retrieved and from the Swedish register of Education completed years at school and finally from the Swedish Cause of Death Register data on causes of deaths .

Time periods - pregnancy length, the trimesters and postpartum. The follow-up period is from the estimated start of pregnancy until day 42 after delivery, i.e. six weeks postpartum or until an event of PE or other VTE occurred before the end of that period. When determining the trimesters, the best estimation of pregnancy length from the MBR that estimates the days of pregnancy by primary ultrasound will be used or when this is missing by calculation of the first day of the last period. Start of pregnancy is defined as the date of delivery minus the precalculated best estimation of the pregnancy length found in the MBR. Regarding trimesters and postpartum the first trimester is defined to comprise the start of pregnancy, day 0 until day 90, the second trimester from day 91 until day 181 and the third trimester from day 182 until three days before delivery date. Delivery and postpartum period is defined to comprise two days before until six weeks after delivery date, since labor often starts 1-2 days before the actual delivery. Women with pregnancy length exceeding 300 days are to be excluded, considered as extreme values.

Exposures - IVF with fresh or frozen embryo transfer The exposures are either an IVF pregnancy with fresh embryo transfer performed directly after ovarian stimulation or an IVF pregnancy with frozen embryo transfer, which was thawed and transferred in a later, non-stimulated cycle. These exposures will be compared to a control group of non-exposed women which consists of all pregnant women not listed in the MBR or the IVF-registries and thus considered to be spontaneously conceived, here referred to as natural pregnancy.