Oocyte aneuploidy mechanisms are different in two situations of increased chromosomal risk: older patients and patients with recurrent implantation failure after in vitro fertilization.

Objective: To clarify the mechanisms underlying oocyte abnormalities in meiosis: meiotic nondisjunction of a whole chromosome or premature separation of sister chromatids in two situations of increased chromosomal risk.

Design: Preconception diagnosis by first polar-body analysis in two situations of increased chromosomal risk.

Setting: Departments of reproductive biology, cytogenetics, gynecology, and obstetrics.