Final answer:
Uterine contractions during labor are primarily stimulated by oxytocin and prostaglandins, which work in a positive feedback loop to enhance contractile strength resulting in childbirth.
Step-by-step explanation:
The uterine contractions during labor and parturition are primarily stimulated by two agents, oxytocin and prostaglandins. As the posterior pituitary increases secretion of oxytocin the hormone stimulates contractions that are intensified by the increasing sensitivity of the uterine muscle or myometrium to oxytocin.
Prostaglandins, which are secreted by fetal membranes and further increased by fetal oxytocin also play a crucial role in enhancing uterine contractions. The combined action of these agents in a positive feedback loop helps in the expulsion of the fetus and placenta during birth.