The good, the bad and the ugly of luteal phase stimulations

An early follicular phase start of ovarian stimulation in assisted reproductive technology (ART) is only required if a fresh embryo transfer is planned. A shift from fresh to frozen embryo transfers has recently characterized ART treatments and, combined with the trend towards treatment individualization and simplification, facilitated random-start stimulation. Luteal phase stimulation, started between ovulation and the next menses, has gained momentum and the good, the bad and the ugly sides have become obvious with the increasing number performed. Unprotected intercourse during the follicular phase or around ovulation can result in an unknown and undetectable conception at the time of starting stimulation. Aside from the theoretical implications for embryo development from exposure to stimulation medication, embryo-derived human chorionic gonadotrophin may cause ovarian hyperstimulation syndrome. The duration of stimulation and consumption of gonadotrophin appear to be longer and higher than in the early follicular phase start approach, although the number of retrieved/mature oocytes is comparable or, in some instances, higher. On the other hand, elevated progesterone concentrations during the luteal phase may prevent premature ovulation and, in theory, might replace pituitary suppression using gonadotrophin-releasing hormone antagonists or exogeneous progestins. Furthermore, the flexibility in stimulation timing will meet the needs of patients with time constraints.