Updated by: Arabinda Naik
Chorion is an extra embryonic membrane which develops from trophoblast. The chorionic cells of placenta secrete a hormone called human chorionic gonadotropin (HCG). It conducts several functions such as: Maintains corpus luteum. Stimulates secretion of progesterone by the corpus luteum. Maintains the endometrial lining of the uterus and helps in its growth during pregnancy. By the 16th week of pregnancy, the placenta begins to produce enough progesterone and the corpus luteum regresses. These functions of hCG are necessary for creating an environment suitable for the developing embryo. As a result of this increased production, hCG accumulates in the maternal bloodstream and is excreted in the urine. Implantation is complete by the middle of the second week. Just a few days after implantation, the trophoblast secreted enough hCG for an at-home urine pregnancy test to give a positive result.