Controlled ovarian stimulation (COS) for in vitro fertilization-embryo transfer (IVF-ET) induces dramatic changes in the hormonal profile of the menstrual cycle. When participants were analyzed according to the gonadatrophin-releasing hormone (GnRH) analogue used for pituitary down-regulation, significantly higher serum progesterone (P) levels were observed with GnRH agonists versus antagonists. Patients with lower P levels demonstrated significantly better ongoing pregnancy rates compared with those showing higher levels of P, and this remained the case when pregnancy rates were corrected for confounding factors, including female age and body mass index (BMI). It is been suggested that serum accumulation of human chorionic gonadotropin (hCG) arising from human menopausal gonadotropin (hMG) could be responsible for elevated P levels. The high serum P levels on the day of hCG administration are a frequent event in GnRH agonist and antagonist IVF/intracytoplasmic sperm injection (ICSI) -ET cycles and is associated with a decreased pregnancy rate.