The development of the early embryo
The embryo arises over the course of the first day from the fertilised ovum through the fusion of the mother’s and the father’s pronuclei. The embryo begins to divide and under optimal conditions it reaches the four-to-eight cell stage after two or three days. In the following days, the embryo then significantly picks up speed: On the fifth day, it consists of over 60 to 100 cells and is in what is known as the blastocyst stage.
The blastocyst culture
The second or third day of embryo development has proven to be a good time for the embryo transfer in IVF or ICSI treatment. Due to medical progress, it has become possible to cultivate embryos for five days until the blastocyst stage.
The later replacement of embryos on day five makes it possible to observe the development of the embryos for longer. Due to the longer time between ovum collection and embryo transfer, the body additionally has more time to recover from the hormonal treatment. A blastocyst thereby often has a higher probability of implanting and thereby leading to a pregnancy.
Blastocyst cultivation is most frequently carried out when many fertilised ova are available. If an embryo reaches the blastocyst stage, then the protective shell (which is known as the zona pellucida) that surrounds it is very thin. The assisted hatching procedure is therefore not carried out on embryos in the blastocyst stage. In the case of ova that were cryopreserved, a blastocyst cultivation is also possible, but is only rarely carried out.
The blastocyst culture increases the change of a successful pregnancy, especially when the patient’s body has produced many mature ova.
A blastocyst has a very high probability of implanting (about 35%) and thereby leading to a pregnancy. This is the reason why, in many countries, there is a preference to continue the cultivation of the embryo until the blastocyst stage.
Due to the high probability of pregnancy during the transfer of embryos in the blastocyst stage, normally two - or sometimes even only one - embryos are transferred. In this way, triplet pregnancies can be almost completely excluded and twin pregnancies can be significantly reduced.
Unfortunately, only about a third of all fertilised ova develop into a blastocyst at all. In many countries that is not a problem. A minimum number of fertilised ova (five, as a rule) are all permitted to continue to grow and on the fifth day, usually, at least one blastocyst is present.
In Germany it is legally prescribed (see Embryo Protection Act www.gesetze-im-internet.de/eschg), that a maximum of three fertilised ova may continue to develop to become embryos. These must be selected already on the day after ovum collection, which is to say before the first cell division.
Longer cultivation until the blastocyst stage therefore often increases the chances of a pregnancy but unfortunately does not provide any guarantee.
As a general rule the additional costs of a blastocyst culture are not paid by the statutory health insurance companies.
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