One of the hormones released by the placenta is human chorionic gonadotropin (hCG). This hormone is secreted by the trophoblast cells even before they become the chorion, and is the hormone assayed in the pregnancy test. Because its action is almost identical to that of luteinizing hormone (LH), hCG maintains the mother's corpus luteum. The corpus luteum, in turn, continues to secrete oestrogens and progesterone, thereby preventing menstruation and further ovulations. At around $10^{\text {th }}$ week, the secretion of human chorionic gonadotropin (hCG) by placenta declines, and the corpus luteum regresses as a result. However, menstruation does not occur because placenta itself secretes oestrogens and progesterone. In fact, the amounts of these two hormones secreted by the placenta far exceed the amounts that are ever secreted by the ovaries. The high levels of oestrogens and progesterone in the blood during pregnancy continue to inhibit the release of $\mathrm{FSH}$ and $\mathrm{LH}$, thereby preventing ovulation. They also help maintain the uterus and eventually prepare it for labor and delivery, and they stimulate the development of the mammary glands in the preparation forlactation after delivery.