Final answer:
The uterus is the organ that involutes after the delivery of a baby, undergoing involution to return to its pre-pregnancy size.
Step-by-step explanation:
The organ that involutes following the delivery of a baby is the uterus. The uterus is a muscular organ shaped like an upside-down pear, which provides a place for the fetus to grow and develop during pregnancy. After childbirth, the uterus undergoes a process called involution, where it returns to its pre-pregnancy size and shape. This process is made possible by the contraction of the uterus's muscular walls, which helped in pushing the baby out during birth.