Which hormone is primarily responsible for the maintenance of pregnancy during the first trimester?

Progesterone is the primary hormone responsible for maintaining pregnancy during the first trimester. It plays a crucial role in developing and maintaining the uterine lining, making it suitable for the implantation of the fertilized ovum. Progesterone also helps to suppress uterine contractions, reducing the chance of miscarriage during the early stages of pregnancy.

In addition to its effects on the endometrium, progesterone is essential for the proper development of the placenta, which further sustains the pregnancy. The corpus luteum secretes progesterone until the placenta takes over this function, typically around the end of the first trimester. This transition is vital for sustaining the pregnancy and ensuring the embryo's proper growth and development.