Progesterone is primarily responsible for maintaining pregnancy by preparing and sustaining the uterine lining (endometrium) for embryo implantation and preventing uterine contractions that could lead to miscarriage. It supports the development of the placenta and modulates the maternal immune response to tolerate the fetus. Unlike estrogen, which mainly promotes uterine growth, progesterone ensures a stable environment for fetal development throughout pregnancy. Although hCG supports early progesterone production by the corpus luteum, progesterone itself is the key hormone for pregnancy maintenance.

Reference: Williams Obstetrics, Cunningham, 25th Edition.