Fertilization usually takes place in the ampulla, the widest section of the fallopian tubes. A fertilized egg immediately starts the process of embryogenesis (development). The developing embryo takes about three days to reach the uterus, and another three days to implant into the endometrium. It has usually reached the blastocyst stage at the time of implantation: this is when pregnancy begins. The loss of the corpus luteum is prevented by fertilization of the egg. The syncytiotrophoblast (the outer layer of the resulting embryo-containing blastocyst that later becomes the outer layer of the placenta), produces human chorionic gonadotropin (hCG), which is very similar to LH and which preserves the corpus luteum. The corpus luteum continues to secrete progestoerone and estrogens at levels slightly above those at ovulation during the first few months of pregnancy. After this and for the rest of the pregnancy, the placenta secretes high levels of these hormones along with human chorionic gonadotropin (hCG), which stimulates the corpus luteum to secrete more progesterone and estrogens, blocking the menstrual cycle. These hormones also prepare the mammary glands for milk production.