Human Chorionic Gonadotropin is the major hormone secreted by the placenta. In the early stages of pregnancy, continued survival of the corpus luteum (the follicle that releases the egg) is totally dependent on HCG and, in turn, the survival of the pregnancy is dependent upon hormones secreted by the corpus luteum during the 7th week of pregnancy. Thereafter, the corpus luteum is gradually replaced by placental hormone secretions. Studies have shown that after administering HCG intramuscularly, ovulation occurs 38 to 40 hours afterwards. This is why during an IVF cycle, egg retrieval is scheduled 30 to 40 hours after the HCG shot. Similarly, during simple ovulation induction with Clomiphene or Gonadatropins, HCG is sometimes given to trigger ovulation. HCG is chemically very similar to LH or luteinizing hormone, which signals the ovulation process to begin. Insemination or intercourse is then planned according to the time that HCG was administered.