Comparison of Blood Flow in the Arteriae Uterinae in Ovarian Stimulation Cycles

Inclusion Criteria: * Patients who undergo ovarian stimulation in a Gonadotropin-Releasing-Hormone (GnRH)-antagonist protocol for IVF / ICSI * Patients who have vitrified embryo(s) * Preparation for FET either in HRT or NC cycle Exclusion Criteria: * Poor responder according to Bologna criteria (Ferraretti et al.) as follows: * At least two of the following three features must be present: * (i) Advanced maternal age (≥40 years) or any other risk factor for poor ovarian reserve (POR); * (ii) A previous POR (≤3 oocytes with a conventional stimulation protocol); * (iii) An abnormal ovarian reserve test (i.e. antral follicle count (AFC) 5-7 follicles or anti-mullerian hormone (AMH) 0.5 -1.1 ng/ml). * Uterine surgery for removal of fibroids (hysteroscopic, laparoscopic) or removal of uterine septum * Endometriosis * Asherman-Syndrome * Previous cytotoxic treatment * Previous radiation of the uterus / adnexal region * Known hypertension * Intake of Aspirin or similar medication which might influence the blood flow * Status after tubal ligation * Status after surgery in the adnexal region on 1 side