Which of these is not an important component of initiation of parturition in humans ?

The correct option is Release of Prolactin.

To understand why this is the correct answer, let us analyze the physiological events that lead to the initiation of parturition (the process of giving birth) in humans and compare them with the functions of the hormones listed in the options:

1. Estrogen to Progesterone Ratio:
During pregnancy, progesterone helps maintain uterine quiescence by keeping the uterine smooth muscle (myometrium) relaxed. Towards the end of gestation, there is a significant increase in the estrogen to progesterone ratio. This shift increases the excitability of the myometrium, promotes the formation of gap junctions between uterine muscle cells, and upregulates the expression of oxytocin receptors, making the uterus highly sensitive to contractile signals.

2. Synthesis of Prostaglandins:
Prostaglandins are local hormones synthesized in the uterus and fetal membranes. They play a critical role in cervical ripening (softening, thinning, and dilating the cervix) and act as powerful stimulators of uterine smooth muscle contractions. Their synthesis is essential for the progression of labor.

3. Release of Oxytocin:
Oxytocin is a hormone synthesized in the hypothalamus and released from the maternal posterior pituitary gland. It stimulates powerful uterine contractions during labor. The physical stretching of the cervix triggers a neural reflex (known as the Ferguson reflex) that stimulates further oxytocin release, creating a positive feedback loop that drives the birth process.

4. Release of Prolactin:
Prolactin is a hormone synthesized and secreted by the anterior pituitary gland. Its primary physiological role is to stimulate the development of mammary glands and promote milk production (lactogenesis). While prolactin levels rise during pregnancy, its release is not involved in initiating or maintaining the uterine contractions that drive parturition.

Therefore, the release of prolactin is not an important component of the initiation of parturition in humans.