Only the highest quality embryos are selected for freezing, usually only those that develop to the blastocyst stage (5 or 6 day old embryos). It is very rare to freeze embryos in a lower stage of development.

In the past, the embryos were frozen slowly, leading to more damage and lower survival rates upon thawing. Today, this is done through a process, called vitrification . Embryos are frozen extremely quickly in liquid nitrogen at a temperature of -196°C. 

Embryos are stored in straws or sheaths, maximally two embryos together in one straw. 

When is FET performed?

• When the transfer of a fresh embryo, obtained during the IVF procedure, is unsuccessful – if the couple has frozen excessive embryos left, only these are transferred in future procedures, and they only enter the fresh IVF procedure (where the puncture is performed again) only when all the frozen ones are used up embryos.
• When a couple wants another child and had frozen embryos left over from previous IVF procedures.
• When a woman develops an acute illness during a fresh IVF procedure.
• When a woman develops a severe form of ovarian hyperstimulation syndrome (OHSS) during a fresh IVF procedure and a possible pregnancy would threaten her health.
• When during a fresh IVF procedure, an abnormality is discovered in the uterus that would have a negative impact on conception and/or pregnancy (e.g. appearance of polyp(s), increased growth of myoma, formation of a new myoma, inflammation, etc.).
• When the lining of the uterus does not reach a satisfactory thickness (7 mm and more) and the probability of success – conception is significantly reduced.
• In the event that the couple decides on pre-implantation genetic diagnostic (PGD).
• In case the couple receives a donated embryo. 

How is a frozen embryo transferred?

The transfer itself takes place in the same way as the transfer of a fresh embryo, but the methods of preparing the woman or uterus to receive an embryo differ from the protocols used in fresh IVF procedures:

1. In spontaneous cycle in women with regular cycles. The onset of ovulation is monitored, usually with a combination of urine ovulation tests (LH tests) and ultrasound examinations, where the growth of the leading follicle is monitored. Once ovulation is confirmed, the date for embryo transfer is set – usually 5 days after ovulation or as old as the embryo was at the time of freezing.

2. With estrogen therapy (Estrofem) in women with irregular cycles or no menstruation. The goal is to thicken the uterine lining and create favorable conditions for the embryo to implant. Embryo transfer is performed 7 days after the mucous membrane reaches the optimal thickness (at least 7 mm). In the event that the mucous membrane does not increase in thickness as expected, the therapy is continued for a longer time, the dose is increased or the procedure is even interrupted.

3. With minimal stimulation of the ovaries – in such a protocol, an oral drug such as Femara or low doses of gonadotropins in the form of subcutaneous injections, such as Gonal F, can be used. The goal is to stimulate follicle growth and finally trigger ovulation with a stop injection. After the stop injection, the date for embryo transfer is determined – usually 6-7 days later.

Transferring embryos during a woman’s spontaneous cycle is therefore only suitable for women with regular ovulation or regular menstrual cycles, without hormonal imbalances or other problems.

In protocols where frozen embryos are transferred with the help of Estrofem, Femara or gonadotropins, shortly before or at the time of transfer, progesterone support therapy is included.