Relaxin is a hormone first recognized by zoologist Frederick Lee Hisaw in his study of gophers. When Hisaw noted the difference in the female and male pelvic girdle in gophers, he found ovarian hormones released during female puberty to be the cause. Since Hisaw’s discovery in 1926, scientists have linked the hormone relaxin to several other female reproductive functions in both humans and animals, and have even found it to be present in male human semen.
One major function of relaxin, and the source of its name, is its ability to relax the pelvic ligaments of pregnant females to prepare for childbirth. It is secreted by the corpus luteum, placenta, decidua, breast, and chorion. Relaxin also relaxes and softens the cervix at the end of the third pregnancy trimester by breaking down collagen. It is vital that the pelvic region and birth canal become more pliant to allow the fetus to pass through during labor, so this hormone is a key component in the birthing process. Relaxin also prevents premature labor by inhibiting uterine contractions before a fetus is ready for birth.
Unfortunately, due to the high concentrations of relaxin required to loosen the pelvic joints during pregnancy, other joints can be affected as well. This can cause pain and inflammation in some women. However, once the pregnancy has ended, relaxin levels will be reduced and the pain will dissipate.
Relaxin is also present in females who are not pregnant, though at much lower concentrations, and it plays a role in female menstruation. Relaxin levels rise towards the end of the female menstrual cycle and this softens and opens the cervix to allow the uterine lining to pass through when it is shed during a woman's period. When menstruation occurs and no pregnancy is present, relaxin levels reduce, only to rise again after ovulation. They work with estrogen and progesterone to regulate the menstrual cycle.
Scientists are studying relaxin's effect on joints and ligaments during the menstrual cycle. It is believed that the relaxing, or loosening, effect this peptide hormone and estrogen have on muscles and ligaments during the menstrual cycle makes women more prone to injury at certain times. The drop and rise of these hormones causes weakness that has been directly attributed to muscle injury. Awareness of this allows female athletes and others to schedule training sessions and activities around the days when weakness is expected.