How does severe endometriosis affect embryo quality?

This is an area where confident claims outrun the evidence, and the answer should reflect that. What is reasonably established: women with endometriosis undergoing ART often have fewer oocytes retrieved, particularly after endometrioma surgery, which removes functional ovarian tissue. Reduced ovarian reserve in this group is largely a consequence of disease and surgery rather than of an intrinsic defect in oocyte quality. What is less established: whether endometriosis independently impairs oocyte or embryo quality once oocyte number is accounted for. Proposed mechanisms exist — altered follicular fluid environment, oxidative stress, inflammatory mediators — but the clinical data are inconsistent, and studies are confounded by prior surgery, reserve and age. A useful observation for counselling: oocyte donation studies, where donor oocytes are placed into recipients with endometriosis, have been used to separate oocyte factor from endometrial factor. Results across this literature have not been uniform, and it remains contested. The practical position: counsel on likely oocyte yield, which is predictable and matters, rather than asserting an embryo quality deficit that is not clearly demonstrated. Where yield is the limiting factor, oocyte accumulation across cycles is the strategy with support. Reviewer: this answer deliberately does not quote effect sizes. Previous content in this corpus stated figures without traceable sources. If specific numbers are wanted, they should be cited to identified studies.