Preparing and Timing of the Endometrium in Modified Natural Cycle Frozen-thawed Embryo Transfers

Inclusion Criteria: * Female age 18-41 years, regular menstrual cycle (23-35 days), vitrified blastocysts derived from 1.-3. IVF/ICSI cycle in a public hospital and undergoing single blastocyst transfer. Exclusion Criteria: \- Previous participation in the study, uterine malformations, intrauterine polyps or submucosal myomas, breast feeding, oocyte donation, preimplantation genetic testing, blastocyst conceived with sperm from testicular sperm aspiration, HIV (woman), hepatitis B and C (woman), known luteal phase insufficiency or if patients are not fulfilling the inclusion criteria. Further exclusion criteria are the following contraindications to progesterone; allergy to the study medication, undiagnosed vaginal bleeding, current missed abortion or ectopic pregnancy, hepatic insufficiency or severe hepatic disease, genital or breast cancer, arterial or venous thromboembolism, thrombophlebitis or porphyria. For patients participating in the sub-study, thyroid disease is an exclusion criterion.