Final answer:
Relaxin and progesterone lead to the weakening of ligaments, with relaxin increasing elasticity for childbirth, and a decrease in progesterone levels towards the end of pregnancy reducing its inhibitory effect on uterine contractions, thus aiding labor onset.
Step-by-step explanation:
Relaxin and progesterone cause a weakening of ligaments. Relaxin, secreted by the corpus luteum and then by the placenta, plays a key role in preparing the body for childbirth by increasing the elasticity of the symphysis pubis joint and pelvic ligaments. This hormonal effect allows for the expansion necessary to accommodate a growing fetus and facilitates the dilation of the cervix during labor.
Progesterone, on the other hand, is known for its role in maintaining pregnancy by inhibiting uterine contractions in the early stages. As pregnancy progresses, however, the levels of progesterone decrease, which reduces its inhibitory effect on the uterus, thereby allowing uterine contractions to increase, contributing to the onset of labor.