Which hormone is crucial for maintaining pregnancy until the placenta is established?

Progesterone is essential for maintaining pregnancy until the placenta is fully developed and can take over hormone production. During the early stages of pregnancy, progesterone is produced primarily by the corpus luteum, which is a temporary endocrine structure in the ovary. This hormone helps to maintain the uterine lining, making it suitable for the implantation of a fertilized egg.

Progesterone also inhibits uterine contractions, thus preventing premature labor and allowing the developing embryo to grow safely in the uterine environment. Once the placenta is established, it begins to produce sufficient progesterone, allowing the corpus luteum to degenerate.

Other hormones listed play distinct roles in the reproductive process. For instance, estrogen is important for the development and maintenance of female reproductive tissues but does not specifically maintain the early stages of pregnancy. Luteinizing hormone (LH) plays a role in ovulation and the formation of the corpus luteum, while human chorionic gonadotropin (hCG) is important for signaling the corpus luteum to continue progesterone production in the early stages of pregnancy but does not maintain pregnancy itself. Therefore, progesterone is the pivotal hormone that sustains pregnancy prior to the placenta's adequate function.