Cryopreservation of embryos and oocytes has become routine in assisted reproduction technology (ART). Live birth rates following frozen embryo transfers have increased significantly. Data show that cryopreservation has significant improved survival rates and cumulative pregnancy rates, as well as the safety of ART . Embryo cryopreservation has become a cornerstone in ART. With improved vitrification techniques. Frozen embryos transfer (FET) shows equal or even higher implantation and pregnancy rates than do fresh embryo transfers ART Previous studies show equal or even superior outcomes regarding pregnancy and live birth rates with frozen versus fresh embryo transfer Cryopreserve all embryos have increased substantially in recent years, and according to the recent trend of a freeze-all strategy. This study aims to answer a clinical question encountered in daily practice regarding at what stage embryos should be frozen.

Cryopreservation of embryos and oocytes has become routine in assisted reproduction technology (ART). Live birth rates following frozen embryo transfers have increased significantly. Data show that cryopreservation has significant improved survival rates and cumulative pregnancy rates, as well as the safety of ART. Embryo cryopreservation has become a cornerstone in ART. With improved vitrification techniques. Frozen embryos transfer (FET) shows equal or even higher implantation and pregnancy rates than do fresh embryo transfers ART .

Previous studies show equal or even superior outcomes regarding pregnancy and live birth rates with frozen versus fresh embryo transfer .

Cryopreservation has several advantages. It maintains supernumerary embryos not used for fresh transfer; allows single-embryo transfer, thus reducing multiple gestations; enables a freeze-all strategy to prevent ovarian hyper stimulation syndrome; is useful for social or medical fertility preservation; and allows embryo biopsy for preimplantation genetic testing, luteal phase stimulation, and dual stimulation protocols .

A recent trend is to perform blastocyst fresh/frozen single-embryo transfers. The advantages include exposing the embryo to a more natural uterine environment. Also, by extending the duration of culture, embryo self-selection will occur and may enable the highest chance of implantation . Due to a potential damage to the expanded blastocyst during vitrification procedure, an emerging clinical question is whether cryopreserving cleavage stage embryos, then thawing and culturing to blastocysts will achieve better outcomes, as compared to transfer of a thawed blastocyst .

The most recent study demonstrated that thawing cleavage embryos, then culturing and transferring them as blastocysts, yields improved pregnancy rates and perinatal outcomes compared to thawed blastocyst embryo transfers . Cryopreserve all embryos have increased substantially in recent years, and according to the recent trend of a freeze-all strategy. This study aims to answer a clinical question encountered in daily practice regarding at what stage embryos should be frozen.

Inclusion Criteria:

1. Patients with history of primary or secondary infertility.
2. Age ≤ 38 years.
3. Female patients having more than 4 vitrified embryos.
4. BMI less than 30 kg/m².
5. Good quality of embryos

Exclusion Criteria:

1. Severe male factor infertility.
2. Recurrent implantation failure.
3. Females with uterine Congenital anomalies.
4. Bad quality of embryos.

Women who will transfer frozen embryos