During early pregnancy, which structure is primarily responsible for secreting progesterone?

During early pregnancy, the primary structure responsible for secreting progesterone is the corpus luteum. After ovulation, the follicle transforms into the corpus luteum, which produces progesterone to maintain the uterine lining and support an early pregnancy. This hormone is crucial because it prepares the endometrium for potential implantation of a fertilized egg and helps sustain the pregnancy during its initial stages.

Although the placenta will take over progesterone production later in pregnancy, it is the corpus luteum that plays the critical role in the first trimester. The trophoblasts, which are cells forming part of the outer layer of the blastocyst, contribute to some hormone production but are not the primary source of progesterone. The endometrium, while essential for implantation and maintenance, does not produce progesterone directly; it is the target site where the hormone exerts its effects. Thus, the corpus luteum's role in early progesterone secretion is fundamental for embryo development and pregnancy continuity during those first crucial weeks.