One of the most common activities performed on every fertility clinic or assisted reproduction unit is embryo transfer with cryopreserved embryos. It represents 20% of the work performed on these centres.
Decisive factors for embryo transfer
When an embryo is about to be transferred into the maternal womb, a series of factors should be taken into account in order to be able to control the process and, therefore, optimise the reproductive process. The aim is to guarantee that pregnancy occurs. The following factors are decisive to perform an embryo transfer:
- Mother's age on the date of embryo cryopreservation
- Cause for sterility
- Embryo quality
- Embryo freezing-thawing technique used
- Optimisation of the endometrium on the date of embryo transfer into the uterus
Natural cycle vs. Replacement cycle
There are different strategies for achieving the last factor mentioned above. These strategies can be classified in two big groups or cycles:
In the natural cycle, advantage is taken of the natural endometrial cycle due to the effect of the endogenous oestrogens and the progesterone secreted by the follicle, which are synthesised by the spontaneous ovarian cycle in cases where it is preserved.
Another option is the replacement cycle, in which the endometrium is prepared through exogenous administration of oestrogens in order to better control the growth and optimise its conditions for the transfer.
Some researchers prefer using natural cycles, since the endometrium becomes "adequate and receptive". A higher number of pregnancies is, a priori, achieved if compared to the number obtained after replacement cycles, even with low levels of estradiol (predominant oestrogen on women's early reproductive years) and equal or greater endometrial thickness. But even after that, there is controversy regarding which procedure is better for preparing the endometrium once the cryopreserved or donated embryo has been transferred. For this reason, techniques such as ultrasound measurement of endometrial thickness, whose aim is to get a higher reproductive success rate, are taken into account.
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