Final answer:
Nursing the newborn promotes uterine involution due to the release of oxytocin, which causes uterine contractions aiding the uterus in returning to its pre-pregnancy size and helps prevent postpartum hemorrhage.
Step-by-step explanation:
Nursing the newborn promotes uterine involution because the act of breastfeeding initiates a hormonal response that stimulates uterine contractions, aiding in the process whereby the uterus returns to its pre-pregnancy size. When an infant suckles at the breast, it triggers the release of the hormone oxytocin. Oxytocin is crucial for milk ejection during breastfeeding, but it also causes the myometrium, the muscular layer of the uterine wall, to contract more strongly.
These contractions help in compressing the blood vessels and reducing bleeding, as well as in expelling any remaining placental fragments, which is vital for preventing postpartum hemorrhage. Another aspect of breastfeeding that facilitates involution is the reduction of estrogen levels, as high levels can inhibit involution. Therefore, while breastfeeding does lead to the formation of maternal antibodies that benefit the infant's immune system, this is not the direct mechanism by which nursing promotes uterine involution.