The use of estrogen in Assisted Reproduction is very common. Estrogens, as female hormones, have the main capacity to thicken the endometrium (inner layer of the uterus). The endometrium must have favorable conditions to favor the implantation of the embryo.
In Artificial Insemination treatments, either in a natural cycle (without medication) or in a substituted cycle, and in In Vitro Fertilization treatments, the activity of the ovary itself indirectly produces estrogens that thicken the endometrium and it is not necessary to add them exogenously on a regular basis. In some cases, it can be observed that during the follicular development the endometrial thickness or the optimal aspect is not reached, being advisable to add estrogens in low doses to try to increase and improve the endometrial development. If added, they should be maintained until the result of B-HCG at least.
Frozen embryo treatments, oocyte or embryo donation deserve a separate mention. In these treatments, when carried out in a substituted cycle (with medication), the hormonal secretion of the ovary must be simulated. In this way, treatment is initiated with the patient's period and estrogens are added in tablets or patches to promote endometrial growth. Normally, a control ultrasound is performed after 10-12 days to check this growth. If the appearance is trilaminar and the thickness is above 7-8 mm, the endometrium is considered to be ready for the embryo transfer. For this, progesterone should be added as many days before the embryo to be transferred. Both hormones (estrogen and progesterone) must be maintained at least until the day of the pregnancy test and if it is positive, the first weeks of gestation will be maintained.