The endometrium in a controlled ovarian stimulation (COS) cycle may be impaired and a suspected decrease in implantation rate has been hypothesized.Endometrial window of implantation implies a fine regulated equilibrium of estradiol and progesterone in terms of circulating levels and timing.In COS there is a supraphysiological level of estradiol, and progesterone may appear earlier despite the use of GnRH analogs to prevent premature LH surge.Although histological evaluation of the endometrium is not a reliable tool, it has been describe an endometrial advancement in the context of COS.Modem molecular techniques, such mRNA expression arrays has allowed to demonstrate that RNA expression profile of endometrium is advanced in COS when progesterone circulating levels exceed a threshold level of 1.5 ng/ml, thus implantation rate is reduce.Moreover, even if progesterone remains below this threshold, endometrial decidualization may begin earlier due to increased sensitivity of endometrial progesterone receptors.In clinical situation where the endometrium is obviously impaired, such as a failure to grow or cven a desquamated one, cryopreservation is mandatory.Controversy appears when serum progesterone is elevated beyond a threshold or when a suspected displacement of the window of implantation is suspected without any other clinical sing.New cryopreservation techniques, specifically vitrification, yield an extraordinary survival rate and clinical results.Here we show that ongoing pregnancy rates are increased in cycles where all embryos were vitrified and transferred in a deferred cycle.