During early pregnancy, which structure primarily produces progesterone to maintain the pregnancy?

During early pregnancy, the primary source of progesterone is the corpus luteum. After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone and estrogen. Progesterone plays a crucial role in preparing the endometrium for potential implantation of the fertilized egg and helps maintain the uterine lining throughout the first trimester. If pregnancy occurs, the corpus luteum continues to produce progesterone until the placenta takes over this function around the end of the first trimester.

While the placental cytotrophoblast does contribute to progesterone production later on in pregnancy, it is not the primary source during the very early stages. The corpus albicans is a degenerative structure left behind after the corpus luteum ceases to function, and the pituitary gland itself does not produce progesterone; it secretes hormones that help regulate the reproductive cycle but relies on the corpus luteum for progesterone production.