Today we freeze embryos only by using the method called vitrification. This method is more expensive than previously performed slow freezing, but it ensures a greater likelihood of good embryo survival after thawing.
Embryos can be freezed (cryopreserved) theoretically for unlimited period of time.
The advantage of FET is its simplicity which eliminates the ovarian stimulation and egg retrieval, i.e. injections and general anaesthesia. Although the treatment cycles with frozen embryos are not covered by public health insurance, it is generally less costly than IVF+ET treatment. If the couple has embryos frozen, it has a chance to conceive using FET and does not need to undergo the next intensive IVF process.
A pregnancy success rate is slightly lower than in the “fresh cycle” (IVF + ET), but today the total number of children born after FET is high.
FET is planned either in the natural (native) cycle, or with hormonal support, i.e. hormone replacement therapy (HRT).
In natural cycle, FET can be planned only if the woman ovulates in her cycle. It is therefore necessary to monitor ovulation by ultrasound, i.e. perform the folliculometry, to determine the day of ovulation. This requires about 2-4 ultrasound examinations. During ovulation, not only that the follicle releases an egg to be fertilized, which in this case is not needed, but this follicle is hormonally active and prepares the endometrium for implantation of the embryotransferred by us. The embryotransfer is performed 2-5 days after ovulation according to the age of the embryo.
In FET with HRT support the endometrium is being prepared by using estrogens and progestogens. It most often consists only in taking tablets and vaginal capsules. A fewer ultrasound examinations are needed (1-2). Both medications are used until a positive pregnancy test and when pregnancy occurs, both medications need to be used for FET with HRT support until the 12th week of pregnancy as there is no corpus luteum!