 Cryopreserved-thawed embryos are often replaced in an artificial cycle, in which the endometrium is prepared by exogenous estrogen and progesterone with or without the use of GnRH agonist downregulation. The correct duration for exposure to progesterone is still not well established. To date, there are no prospective randomized trials available, comparing different durations of progesterone supplementation before the date of transfer, with regard to treatment outcome Nawroth. 