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Embryo transfer, also known as retransfer, is a very simple procedure from a medical perspective. However, for most couples trying to have children, it is a very special moment during their medical fertility treatment, as this is when the doctor inserts the embryos in the woman’s uterus. It is when the pregnancy can begin.
After the eggs are removed from the woman’s ovary (follicular aspiration), they are immediately fertilised in the laboratory by means of in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI). Then are then put in an incubator and begin to divide. The various stages of development include:
It can often be useful to perform the embryo transfer on the third day after the follicular aspiration, that is, when the embryos are in the eight-cell stage. On the other hand, waiting to perform the retransfer until the blastocyst stage is also referred to as blastocyst transfer.
An embryo transfer takes only a few minutes. It is a routine procedure and can be performed without anaesthesia. In consultation with the parents-to-be, the physician will insert the embryo or embryos into the woman’s uterus in accordance with the legal regulations. To do this, the doctor will use a soft plastic catheter. At this point, the embryos are barely visible to the human eye and are in a tiny droplet of medium – this tiny droplet is inserted completely painlessly into the uterine cavity.
Once the embryos are in the uterus, they can implant in the lining of the uterus where they will continue to develop. The patient can go home immediately after the surgery.
The process of embryo transfer entails virtually no risks. According to the German Embryo Protection Law, the number of embryos capable of development created per cycle may not exceed the number to be transferred to the patient (a maximum of three).
Transferring two ideal embryos leads to pregnancy after IVF therapy in 44 percent of embryo transfers in under-30s, and around 27 percent for 40-year-olds (DIR 2015). Of these, around 22 percent become pregnant with twins.
After embryo transfer of three embryos, on the other hand, up to 27 percent of the pregnant women become pregnant with twins and up to 4 percent with triplets. A multiple pregnancy can, however, be dangerous for both the expectant mother and the children. It is therefore considered to be a high-risk pregnancy. For this reason, triplet pregnancies in particular should therefore be avoided at all costs. In countries with selective blastocyst culture, only one embryo per transfer is therefore inserted in the uterine cavity, and only in cases of reduced pregnancy chances are multiple embryos transferred.
If a pregnancy occurs after an embryo transfer, the pregnancy hormone hCG formed by the pregnancy can lead to hyperstimulation syndrome. This can cause the ovaries to enlarge and can result in water retention in the stomach and chest area. In the worst case, hospitalisation may be necessary. In any case, an individual consultation and explanation will be provided by the treating physician, taking into account your personal circumstances and any concerns you may have.
Excess embryos – i.e. embryos not required for subsequent attempts – are fortunately very rare. If the pregnancy hormone level does not rise following an embryo transfer (negative pregnancy test), it is, however, beneficial for the patient to be able to fall back on cryopreserved (pre-)embryos so that she doesn’t have to repeat stimulation treatment and follicular aspiration. Freezing embryos (cryopreservation) is not normally permitted in Germany under the Embryo Protection Law. However, there may be individual cases in which an embryo transfer cannot be done for certain reasons. In this case, the excess embryos can be cryopreserved for a later attempt.