What hormone is primarily responsible for maintaining pregnancy during the first trimester?

Progesterone is the hormone primarily responsible for maintaining pregnancy during the first trimester. It plays a crucial role in supporting the uterine lining, which is essential for the implantation of the fertilized egg. Progesterone helps to create a favorable environment for the developing embryo and prevents uterine contractions that could lead to miscarriage.

In the early stages of pregnancy, particularly during the first trimester, progesterone is produced by the corpus luteum — the structure that forms in the ovary following ovulation. As pregnancy progresses, the placenta takes over the production of progesterone, ensuring that its levels remain high to support the pregnancy.

While estrogen also plays a significant role in pregnancy by promoting the growth of the uterine lining and the development of the placenta, it is not the primary hormone for maintaining pregnancy in the first trimester. Human chorionic gonadotropin (hCG) supports the corpus luteum's production of progesterone initially but is not directly responsible for maintaining pregnancy. Testosterone is not involved in the maintenance of pregnancy and is primarily associated with male reproductive functions. Thus, the primary hormone that maintains pregnancy during the first trimester is indeed progesterone.