The next step is likely to be; Oxytocin causes the uterus to contract, further stimulating the receptors, and releasing more oxytocin from the brain. Oxytocin is a vital hormone in human that acts on organs in the body, such as breasts and uterus, they act as chemical messenger in the brain, controlling childbirth and lactation, and aspects of human behavior. During childbirth the hormone causes the uterus muscle to contract during labor so that the body can be pushed out.