The first step when a woman undergoes in vitro fertilization (IVF) is ovarian stimulation to produce the development of multiple follicles. Then, when the follicles have reached the desired size, the follicular puncture is performed to obtain the oocytes. These oocytes will be fertilized in the laboratory with the sperm of the couple or of a donor to give rise to the embryos.
Once the embryos have been developed and it is decided to transfer them, it is necessary to perform an endometrial preparation so that it is receptive and favours the implantation of the embryos. This endometrial preparation consists of the administration of estrogens and progesterone.