Final answer:
Uterine involution is the process of the uterus returning to pre-pregnancy size after birth, with oxytocin release during breastfeeding helping this process. Subinvolution is a condition where the uterus fails to properly involute, possibly due to retained placental fragments or infection, contributing to postpartum hemorrhage.
Step-by-step explanation:
Uterine involution is the process by which the uterus returns to its pre-pregnancy size and condition following childbirth. This process involves the contraction of uterine muscles and the reduction of the endometrial lining. Uterine involution is facilitated by the hormone oxytocin, which is released during breastfeeding and stimulates uterine contractions. The presence of a postpartum vaginal discharge called lochia is a part of this process, transitioning from lochia rubra to lochia serosa, and finally to lochia alba as the uterus heals.
Subinvolution refers to the delay or failure of the uterus to return to its normal size after childbirth. This can be caused by several factors, such as retained placental fragments, infection, or poor uterine muscle tone. Retained fragments of the placenta can lead to postpartum hemorrhage and inhibit the normal contraction of the uterus necessary for involution.