What is Relaxin?

Hillary Flynn
Hillary Flynn

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.

In the third trimester of pregnancy, relaxin breaks down collagen and relaxes the cervix.
In the third trimester of pregnancy, relaxin breaks down collagen and relaxes the cervix.

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.

Relaxin levels rise towards the end of the menstrual cycle and reduce after menstruation.
Relaxin levels rise towards the end of the menstrual cycle and reduce after menstruation.

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 plays a role in female menstruation.
Relaxin plays a role in female menstruation.

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.

The hormone relaxin helps ease uterine contractions that could lead to premature labor.
The hormone relaxin helps ease uterine contractions that could lead to premature labor.

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.

Relaxin helps to relax the pelvic ligaments of pregnant women in preparation for childbirth.
Relaxin helps to relax the pelvic ligaments of pregnant women in preparation for childbirth.
Scientists are studying relaxin's effect on joints and ligaments during the menstrual cycle.
Scientists are studying relaxin's effect on joints and ligaments during the menstrual cycle.
High concentrations of relaxin during pregnancy may cause pelvic pain.
High concentrations of relaxin during pregnancy may cause pelvic pain.
Hillary Flynn
Hillary Flynn

Joining the wiseGEEK team has helped to satisfy Hillary’s insatiable curiosity for knowledge. She also manages an electronic publishing business that allows her to develop her skills in technical writing, graphic design, and business development. Hillary has a passion for satirical writing and enjoys traveling, especially to places of historical importance.

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Discussion Comments

TalkingByte

@Glasis - Relaxin does make your joints looser, but frequent urination during pregnancy comes from the pressure applied on your bladder as your baby grows in the womb.

Relaxin's job of loosening the pelvic joints around the bladder probably do contribute at least a little to "leakiness" when you sneeze or cough.

Relaxin is at the root of another common pregnancy problem of lower back pain, however.

Glasis

Is relaxin to blame for weakened bladders in pregnant women, as well?

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