Progesterone Supplementation After Letrozole-stimulated Insemination

This trial aims to investigate whether luteal phase support improves the chance of pregnancy in women undergoing intrauterine insemination (IUI) following ovarian stimulation with letrozole. In this randomized clinical trial, 690 women undergoing letrozole-stimulated IUI at four public fertility clinics in Denmark will be randomly allocated to one of two groups: * Vaginal progesterone from the day after insemination (Cyclogest 400 mg twice daily) * No luteal phase support, reflecting current clinical practice All participants will undergo a standard letrozole-stimulated IUI treatment, including ultrasound monitoring, ovulation triggering, and insemination. Blood samples will be collected on the day of insemination and 7-9 days after insemination to measure hormone levels. The results of this trial will provide further insight into the role of progesterone support in letrozole-stimulated IUI cycles.