Final answer:
During the third trimester of pregnancy, progesterone is produced by the placenta. This hormone plays a critical role in maintaining the pregnancy and preparing the body for childbirth by suppressing uterine contractions.
Step-by-step explanation:
Before pregnancy, progesterone is produced by the ovaries to thicken the endometrial lining and ensure pregnancy. But during the third trimester of pregnancy, the organ responsible for the production of progesterone changes. Initially produced by the corpus luteum, the responsibility for progesterone production eventually transfers to the placenta as the corpus luteum degenerates. The placenta then becomes the primary endocrine organ supporting the pregnancy, producing both estrogen and progesterone, which are crucial for maintaining the health and growth of the fetus and preparing the mother's body for childbirth. Specifically, progesterone plays a key role in preventing the onset of labor by inhibiting uterine contractions and the release of other hormones that could stimulate labor. As the pregnancy progresses toward the end, a decrease in progesterone levels allows for these contractions to increase, leading to childbirth.
Therefore, during the third trimester of pregnancy, progesterone is produced by the placenta (a).