Final answer:
The term used when the fundus descends and the uterus returns to its pre-pregnancy size is involution.
Step-by-step explanation:
The term used when the fundus descends and the uterus returns to its pre-pregnancy size is involution. After giving birth, the uterus undergoes a process called involution in which it gradually shrinks back to its pre-pregnancy size. This process is facilitated by uterine contractions and is important for the mother's recovery and the return of her abdominal organs to their pre-pregnancy locations.