• Estrogen: Progesterone ratio increases during parturition. Progesterone normally inhibits uterine contractions. Near the time of parturition, the ratio rises, triggering labor.

• Prostaglandins: Promote contraction of uterine smooth muscle and cervical dilation.
Oxytocin: From posterior pituitary, stimulates strong uterine contractions (positive feedback loop leading to fetal ejection reflex.
Prolactin: Secreted from anterior pituitary, but its role is in lactation (milk production) after delivery, and not in the initiation of parturition.

• Hence, prolactin is not an important factor in initiating parturition.