Final answer:
The biosynthesis of progesterone in the fetus takes place in the C. placenta during the third trimester of pregnancy as it takes over from the corpus luteum.
Step-by-step explanation:
The biosynthesis of progesterone in the fetus occurs in the C. placenta during the third trimester of pregnancy.
Before pregnancy, progesterone is produced by the ovaries, and this hormone is crucial for thickening the endometrial lining and ensuring pregnancy.
However, as the corpus luteum degenerates, the placenta takes over this role, continuing the production of progesterone, which is essential for maintaining the pregnancy, preventing new ovarian follicles from developing, and suppressing uterine contractility.
This transition ensures a stable supply of progesterone until childbirth.