Hi Vera,
To transfer vitrified embryos, the uterus must be receptive. Therefore, it is essential that it has a trilaminar appearance and a thickness of 7-9 mm. These endometrial characteristics favor the embryo or embryos to implant in the uterus and lead to pregnancy.
For this, most specialists establish a protocol for the administration of estrogens and progesterone for endometrial preparation. However, these drugs are usually started between the first and third day of menstruation. Another alternative would be to induce menstruation with drugs.
In the following article you can get more information: How is the endometrial preparation for embryo transfer?
I hope I’ve helped.
A greeting.
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