Modified natural cycle in in vitro fertilization.

The first live birth after IVF was achieved in a purely natural cycle. Because early attempts at IVF were associated with low efficiency, ovarian stimulation was added to achieve a greater margin for error in oocyte retrieval, fertilization, and thus, overall pregnancy success. As technology improved, the intuitive appeal of the natural cycle led investigators to once again attempt IVF without antecedent gonadotropin stimulation. Triggering of ovulation with hCG was added to allow for accurate scheduling of oocyte retrieval and thus increased oocyte yield. When GnRH antagonists became available, premature ovulations could be prevented, albeit at the cost of adding some form of ovarian stimulation to continue follicle development until ovulation triggering. This type of cycle came to be known as the "modified natural cycle." These modified natural IVF cycles are associated with decreased medication costs, they produce acceptable pregnancy rates, and they may be particularly appropriate for patients at increased risk of ovarian hyperstimulation syndrome, poor responders, and those wishing to avoid supernumerary embryo production.