<P> The corpus luteum is essential for establishing and maintaining pregnancy in females . The corpus luteum secretes progesterone, which is a steroid hormone responsible for the decidualization of the endometrium (its development) and maintenance, respectively . It also produces relaxin, a hormone responsible for softening of the pubic symphysis which helps in parturition . </P> <P> If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 10 days in humans). It then degenerates into a corpus albicans, which is a mass of fibrous scar tissue . </P> <P> The uterine lining (endometrium) is expelled through the vagina (in mammals that go through a menstrual cycle). In an estrous cycle, the lining degenerates back to normal size . </P> <P> If the egg is fertilized and implantation occurs, the syncytiotrophoblast (derived from trophoblast) cells of the blastocyst secrete the hormone human chorionic gonadotropin (hCG, or a similar hormone in other species) by day 9 post-fertilization . </P>

When fertilization and implantation occur what prevents the regression of the corpus luteum