The posterior pituitary hormone that stimulates the strong uterine contractions of true labor, in a positive feedback loop with prostaglandins, is:

1. As labor approaches, the posterior pituitary increases its secretion of a specific contraction-stimulating hormone. 2. The myometrium simultaneously increases its number of receptors for this hormone. 3. This hormone is oxytocin. 4. Oxytocin stimulates prostaglandin release from fetal membranes in a positive feedback loop, which further strengthens contractions, unlike relaxin, which instead loosens pelvic ligaments. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1345 §Physiology of Labor_