Progesterone

After ovulation, the corpus luteum produces progesterone, which transforms the endometrium from a proliferative state to a secretory state — ready to receive an embryo. In early pregnancy, progesterone prevents uterine contractions and supports the developing placenta until it can produce progesterone independently (around 8–10 weeks).

In all IVF cycles, progesterone supplementation begins after egg retrieval (or before the FET transfer) to ensure adequate luteal support. Forms include vaginal suppositories or gel (Crinone, Endometrin), intramuscular injections (PIO), or subcutaneous injections (Rekovelle, Prolutex). Progesterone blood levels are sometimes monitored during FET cycles to ensure levels are adequate on transfer day.