Which of the following hormones is not secreted by the human placenta ?

The correct option is Relaxin.

To understand why this is the correct answer, let us look at the hormones produced during pregnancy and their specific sites of secretion:

1. Human Chorionic Gonadotropin (HCG): This hormone is produced exclusively by the placenta (specifically, the syncytiotrophoblast cells) soon after implantation. It signals the corpus luteum to continue progesterone secretion, maintaining the pregnancy.
2. Progesterone: During the first trimester, progesterone is secreted by the corpus luteum in the ovary. However, by the second trimester, the placenta takes over and secretes large amounts of progesterone to maintain the uterine lining.
3. Estrogen (principally Estriol): Similar to progesterone, the placenta becomes the primary site of estrogen production as the pregnancy progresses, utilizing precursors made by the fetal adrenal glands.
4. Relaxin: In humans, relaxin is primarily secreted by the corpus luteum of the ovary, not the placenta. While it plays a critical role in pregnancy by relaxing the pelvic ligaments and softening the cervix, its primary source of secretion is ovarian rather than placental.

Therefore, among the options listed, Relaxin is the hormone that is not secreted by the human placenta.