Introduction:
The human placenta is a temporary organ that develops during pregnancy and plays a crucial role in the growth and development of the fetus. It is responsible for the exchange of nutrients, gases, and waste products between the mother and the developing baby. The placenta also produces various hormones that are essential for maintaining a healthy pregnancy.

Explanation:
Among the given options, the hormone that is not a secretion product of the human placenta is prolactin. Here's a detailed explanation of why:

1. Human chorionic gonadotropin (hCG):
- One of the earliest hormones produced by the placenta.
- It is responsible for maintaining the corpus luteum, which produces progesterone during the early stages of pregnancy.
- hCG is commonly used as a marker in pregnancy tests.

2. Estrogen:
- Placenta secretes estrogen, particularly estriol, which is the main estrogen during pregnancy.
- Estrogen plays a vital role in the growth and development of the fetus, as well as in maternal adaptations to pregnancy.

3. Progesterone:
- Placenta produces progesterone, which is essential for maintaining pregnancy.
- Progesterone helps in the development of the endometrium, prevents contractions of the uterus, and prepares the breasts for lactation.

4. Prolactin:
- Prolactin is not produced by the placenta but by the anterior pituitary gland in the brain.
- It is primarily responsible for stimulating milk production in the mammary glands after childbirth.

Conclusion:
In summary, while human chorionic gonadotropin (hCG), estrogen, and progesterone are hormones secreted by the human placenta, prolactin is not. Prolactin is produced by the anterior pituitary gland and is responsible for stimulating milk production.