The last step of in vitro fertilization (IVF) is the transfer of embryos. To do this, it is necessary to prepare the endometrium so that it acquires an adequate thickness and a trilaminar aspect, factors that favour embryo implantation. This endometrial preparation can be done in two ways, which is known as natural cycle and substituted cycle.
As can be seen in the image, the endometrial preparation in a substituted cycle is carried out by exogenous drug administration. Specifically, the woman will have to take estrogens and progesterone to simulate the natural behavior of the uterus. Therefore, the woman will start on these drugs on the day that menstruation begins until the endometrium acquires the desired characteristics.