We know that correct endometrial perfusion is key to the growth of the endometrium and therefore to correct embryo implantation. If there is no correct blood supply to the endometrium, even if we administer exogenous hormones, they will never reach it properly. This is the reason why we administer drugs that promote the arrival of blood to the endometrium (sildenafil, pentoxifylline, vitamin A, vitamin E ...) when we are faced with refractory endometrios to treatment with estrogens and progesterone (those that do not grow despite the correct hormone intake).