Yes, endometriosis consists of the presence of endometrium in a place where it does not belong, i.e. outside the uterine cavity.
This endometrium is the tissue on which the embryo implants. It is hormone-dependent, it grows every month influenced by hormones, and when there is no pregnancy, it sheds and menstruation occurs.
In the uterus, there is a natural outlet for it (cervix, through which the tissue and blood reaches the vagina and through it to the outside), however, this does not occur in endometriosis.
Thus, we could say that in endometriosis foci there is also a response to hormones and a small menstruation is generated. Since there is no outlet for it, the blood accumulates, forming what are commonly called "chocolate cysts". That is, cysts filled with oxidized, aged blood that is retained in the endometriosis foci, which are very often ovarian.
Once the endometriotic foci are generated, the woman's body will tend to fight against them, generating adhesions that will try to encapsulate and separate the lesions from the rest of the body, activating the inflammatory/immunological response and releasing substances that are harmful to the oocytes. All this will mean that, in general, the oocytes obtained from patients with endometriosis will have a lower quality than those obtained from patients without endometriosis.
However, this does not mean that a woman suffering from endometriosis cannot have offspring. We must remember that most cases of endometriosis are asymptomatic and do not even require assisted reproductive techniques to fulfill their genetic desire.