Background Assisted reproductive technology (ART) is increasingly used worldwide to treat infertility and assist couples and single mothers with their conception. The most common ART procedure includes treatment with exogenous hormones to stimulate the ovaries to produce multiple oocytes, followed by an in vitro fertilisation (IVF) achieved by letting oocyte and sperm combine in vitro or by direct intracytoplasmic sperm injection (ICSI) into the oocyte. After cultivation into an embryo for two to three days or five to six days into a blastocyst, a subsequent embryo transfer (ET) is performed, either a fresh ET or a frozen-thawed embryo or blastocyst transfer in a later, natural or ”programmed” cycle. During pregnancy the incidence of venous thromboembolism (VTE) is at least five-fold increased as compared to same-aged, non-pregnant women and the incidence is even further increased during ART pregnancies. A doubled incidence has been reported during the whole pregnancy, mostly accounted for in the first trimester by a four-fold increased incidence of overall VTE and a seven-fold increased incidence of pulmonary embolism (PE) in women giving birth after ART as compared to matched women with spontaneous pregnancies. Even though the incidence is low with 1-2 per 1,000 in spontaneous pregnancies and 3-6 per 1,000 in ART pregnancies there has to be an awareness of the risk. VTE and PE are responsible for long-term morbidity, with the post-thrombotic syndrome and impaired exercise capacity after PE, and also an increased mortality with PE being a leading cause of maternal mortality. Since the highest incidence was found during the first trimester of ART pregnancies we hypothesised that this could be related to the hormone increase during ovarian stimulation. Aims The aim of this doctoral project was to achieve more knowledge of potential mechanisms behind the increased incidence of VTE found in ART pregnancies as compared to spontaneous pregnancies. Methods and results To gain knowledge on potential mechanisms explaining the prothrombotic state during ART, we analysed previously collected blood samples from 31 women undergoing ovarian stimulation. The samples were taken at down regulation of the ovaries, when endogenous oestrogen levels are almost undetectable, and during ovarian stimulation with its 10-100-fold increased oestrogen level. We aimed to detect changes of cell-derived plasma microvesicles (MVs) and found a significant increase of potentially prothrombotic MVs with a majority of platelet-MVs exposing markers of activation and inflammation. Further, we analysed alterations of the plasma MV proteome to probe quantitative and qualitative changes of the whole MV protein content during ovarian stimulation. We aimed to capture and identify potential pathophysiological mechanisms concomitant with the oestrogen surge. We further aimed to find potential biomarkers that could help to identify the development of a hypercoagulative state that could help to…