The hormone progesterone is key in maintaining the lining of the uterus. When an egg is released by the ovary, the follicle that the egg developed in, called the corpus luteum, produces progesterone, which causes growth of the uterine lining and suppresses menstruation. The corpus luteum continues to produce progesterone for seven to ten days, and then, if conception has not occurred, it deteriorates and the uterine lining begins to breaks down, resulting in the menstrual flow. However if conception has occurred, the developing placenta begins to secrete human chorionic gonadotrophin, or hCG. The presence of hCG prevents the corpus luteum from disintegration; as a result the corpus luteum continues to create progesterone, preventing menstrual flow, and increasing the thickness of the uterine lining.
About eight weeks after conception the placenta begins to create progesterone, and continues to do so until the end of the pregnancy. The continuing presence of progesterone prevents menstruation.