Final answer:
The hormone called relaxin is responsible for relaxing the pelvic ligaments and pubic symphysis, increasing their elasticity to facilitate childbirth.
Step-by-step explanation:
The hormone responsible for relaxing the pelvic ligaments and the pubic symphysis to prepare for childbirth is called relaxin. Relaxin, which is secreted by the corpus luteum and later by the placenta, increases the elasticity of the symphysis pubis joint and pelvic ligaments. This makes room for the growing fetus and allows for the expansion of the pelvic outlet during childbirth. Relaxin also plays a role in dilating the cervix during labor. Unlike oxytocin, which stimulates uterine contractions during labor, relaxin works to ease the physical changes necessary for childbirth.