Embryo freezing (cryopreservation) is a technique that preserves embryos in liquid nitrogen at -196°C. Originally introduced in the 1980s, it was primarily used to store extra embryos after IVF, reducing the risk of multiple pregnancies by avoiding the transfer of all viable embryos at once.  

Today, embryo freezing is widely used to separate the embryo transfer cycle from the ovarian stimulation cycle, improving pregnancy outcomes. More clinics are also freezing embryos individually rather than in groups to allow for single embryo transfers (SET) in future cycles.  

When Is “Freeze-All” Recommended?  

The “freeze-all” strategy involves freezing all embryos and postponing the transfer to a later cycle. This is recommended in cases such as:  

• High ovarian response (risk of ovarian hyperstimulation syndrome - OHSS).  

• Elevated progesterone levels during stimulation, which may lower implantation rates.  

• Uterine abnormalities such as polyps, fibroids, or hydrosalpinx (fluid-filled fallopian tube), which could impair implantation.  

By delaying the transfer until the uterus is in optimal condition, frozen embryo transfer (FET) can improve success rates.  

At What Stage Can Embryos Be Frozen?  

Embryos can be frozen at any stage of development:  

• Day 2 (4-cell stage)  

• Day 3 (8-cell stage)  

• Day 5 (blastocyst stage)  

Most fertility clinics now prefer freezing at the blastocyst stage, as these embryos have a higher implantation potential, allowing clinics to select the strongest embryos before freezing.  

Does Freezing Affect Embryo Quality?  

No, the duration of freezing does not affect embryo potential. Studies have shown that embryos frozen for more than 15 years have the same success rates as those frozen for shorter periods.