Final answer:
During the third trimester of pregnancy, progesterone is primarily produced by the placenta. This shift from the corpus luteum to the placenta occurs to maintain the pregnancy and support the developmental environment for the fetus.
Step-by-step explanation:
The biosynthesis of progesterone during pregnancy takes place at different sites depending on the stage of the pregnancy. Initially, progesterone is produced by the ovaries, specifically by the corpus luteum. However, as the pregnancy progresses, specifically during the third trimester, the placenta becomes the primary site for progesterone production.
Progesterone plays a critical role in maintaining the pregnancy by suppressing uterine contractions and supporting the endometrial lining to ensure a successful environment for the developing fetus. The shift of progesterone production from the corpus luteum to the placenta typically occurs between the 12th to 17th weeks of pregnancy, as the placenta becomes adequately developed to take over the hormonal functions necessary for pregnancy maintenance.