Final answer:
HCG stands for Human Chorionic Gonadotropin, a critical pregnancy hormone produced by the chorion that maintains the corpus luteum and progesterone levels required for sustaining an early pregnancy.
Step-by-step explanation:
HCG stands for Human Chorionic Gonadotropin. It is a hormone produced by the chorion of the zygote that plays a crucial role during the early stages of pregnancy. This hormone helps to maintain the corpus luteum and ensures elevated levels of progesterone, which is essential for the sustenance of pregnancy. Specifically, hCG is structurally similar to luteinizing hormone (LH) and it preserves the corpus luteum which, in turn, secretes progesterone to suppress menses and create an environment suitable for the developing embryo. Human chorionic gonadotropin is a critical marker used in medical testing to confirm pregnancy, as its presence in a woman's blood or urine is a reliable indicator of pregnancy.