Explanation:
A surge in Leutinizing Hormone casues the release of a mature ovum from the ovary – ovulation.
When the ovum meets a sperm, it is fertilized, forming a zygote. The zygote implants on the thick walls of the uterus (if fertilization had not occurred, the walls of the uterus would be shed by menstruation). The follicle that released the ovum transforms into a Corpus Leuteum. It begins to produce progesterone to maintain the pregnancy until the placenta develops. Estrogen levels remain low.
When the placenta is fully developed, it takes over the role of producing progesterone from the Corpus Leuteum. The zygote develops into a fetus as the placenta nourishes it to full term. At the end of gestation, the fetus exits the uterus via the vagina in the process of giving birth.
When the placenta is also removed, the estrogen levels in the female rise again, and that of progesterone is lowered.
