A frozen embryo transfer, also known as FET is the process of transferring embryos that have been stored. Using frozen embryos allows you to maximise the chance of achieving an ongoing pregnancy from a single IVF or ICSI cycle. Thawing, warming and transferring a cryostored embryo allows you to achieve a pregnancy without the medical risks and costs of a fully stimulated IVF or ICSI cycle.
If possible, thawed frozen embryos are transferred during a woman’s natural cycle. However, women with irregular menstrual cycles may undergo hormone treatment to make the uterus more receptive to implantation.
What is the process of transferring cryostored embryos?
- Before a cryostored embryo can be transferred the lining of the uterus (called the endometrium) has to be thickened to be able to support the embryo. There are a number of ways to prepare the endometrium which your doctor will discuss with you.
- Once the endometrium is prepared, scientists will warm the embryo or embryos. This takes about half an hour, where the scientist will look at the embryo under the microscope to determine if it has survived the warming process.
- An embryo transfer is performed in the transfer room at Care Fertility by your doctor.
What embryos are used?
Some embryos will survive the warming process, some will lose one or more cells, and others will fail to survive altogether. Generally an embryo stored at day 2 or 3 of development is considered to have survived the thawing process if half the cells or more remain after the thawing process. If fewer than half the cells have survived the scientist will thaw another embryo, if available and if you have given permission for this to occur.
If the embryo was stored at day 5 or 6 it is called a blastocyst. When blastocysts are warmed the pocket of fluid within the blastocyst (called the blastocoele) re‐expands over a period of two hours. If re‐expansion of the blastocoele is not seen the pregnancy rate is reduced.