This clinical trial is focused on studying infertility, a condition where individuals have difficulty getting pregnant. The study is investigating the use of vaginal progesterone as a treatment to support the luteal phase, which is a part of the menstrual cycle that occurs after ovulation. The trial aims to determine if using vaginal progesterone can improve the chances of a live birth for women undergoing a specific type of fertility treatment called frozen/thawed in-vitro fertilization natural cycles (FET-NC). Two forms of progesterone are being used in this study: Cyclogest 400 mg vagitorium and Lutinus 100 mg vaginaltabletter.

The purpose of the study is to see if providing luteal phase support with vaginal progesterone is better than not providing any support, in terms of achieving a live birth. If it is found to be beneficial, the study will also compare whether a 7-week treatment is more effective than a 3-week treatment. Participants in the study will receive either the progesterone treatment or a placebo, and their progress will be monitored over time to assess various outcomes related to pregnancy and birth.

Throughout the study, researchers will track several outcomes, including live birth rates, pregnancy progression, and any complications that may arise during pregnancy. The study will also look at the cost-effectiveness of the treatment and measure progesterone levels in blood samples. The trial is expected to continue until early 2026, with the goal of providing valuable insights into the effectiveness of vaginal progesterone in supporting successful pregnancies for those experiencing infertility.