Final answer:
The area of the uterus that flexes during delivery is the cervix, which dilates and effaces to allow the baby to pass through the birth canal.
Step-by-step explanation:
The area of the uterus that flexes during delivery is the cervix. The uterus is a muscular organ with three main sections: the fundus, the body, and the cervix. The cervix is the narrow, lower portion of the uterus, which projects into the vagina. During childbirth, the cervix dilates and effaces to allow the baby to pass through the birth canal. It is composed of strong muscles that push the baby out during birth with forceful contractions. The cervix's mucus secretions, which facilitate sperm movement during fertilization, also change during labor to allow the baby to pass more easily.