Relaxin

Relaxin is a peptide hormone.
Function.
In humans.
Relaxin is produced in females. It rises to a peak about 14 days after ovulation, and then declines if the egg is not fertilised. During the first three months of pregnancy, levels rise and more relaxin is produced. Relaxin adjusts blood supply during pregnancy. It also relaxes pelvic ligaments. It softens the pubic symphysis (the ligaments joining the bottom of the pelvis).
In males, relaxin also has a function: it enhances motility of sperm in semen.
In other animals.
In animals, relaxin widens the pubic bone and facilitates labor. It also softens the cervix (cervical ripening), and relaxes the uterine musculature. Thus, for a long time, relaxin was looked at as a pregnancy hormone. However, its significance may reach much further. Relaxin affects collagen metabolism, inhibiting collagen synthesis and enhancing its breakdown.