What Does the Hormone Relaxin Promote?

Relaxin is found in both men and non-pregnant women in very small amounts. In women, it is produced in the ovaries and in men, the prostate manufactures the hormone.


However, once a woman becomes pregnant, the corpus luteum of the ovaries can no longer manufacture a sufficient amount of the key hormone and it is also produced by the placenta and the endometrial lining of the uterus.

Implantation Assistance

The hormone relaxin relaxes the uterus to prevent contractions while the embryo implants. Its levels start to rise in the mother’s system fourteen days after ovulation when the egg is preparing for implantation. Relaxin also helps the uterus prepare for the advancing pregnancy by building up the endometrial lining. It is one of the key players in ensuring a healthy endometrial lining that will support a full term pregnancy, according to the researchers at the Department of Obstetrics, Gynecology and Women’s Health, New Jersey Medical School of UMDNJ. Overall, the hormone relaxin promotes a healthy pregnancy and an easier delivery.

After Embryo Implantation

Once the embryo implants, the hormone continues to aid in the development of the fetus, especially during the first trimester when the levels of the hormone are at their highest. Typically, a woman will eventually have ten times the amount of the hormone relaxin in their system during their advancing pregnancy compared to their pre-pregnancy state.

Assisting the Mother’s System

As the pregnancy advances, relaxin also protects the mother’s health by regulating her cardiovascular and renal systems so that they can support the increased oxygen requirements of the growing fetus. It also helps the mother’s body regulate the resulting waste products by relaxing the mother’s blood vessels so that ample blood flow to the placenta and the mother’s kidneys.

Mammary Glands

During the second trimester, the pivotal hormone also starts to prepare the mammary glands for lactation. Without adequate levels of the hormone, the mammary glands will not develop sufficiently enough to produce colostrum and milk.

Stretching the Muscles & Ribcage

As the pregnancy advances the growing fetus requires more room so the mother’s ribcage and abdominal muscles must stretch. The hormone assists in relaxing the muscles and ligaments so they readily expand to accommodate the burgeoning girth.

Last Trimester of Pregnancy

In the last trimester, relaxin softens the mother’s cervix and vagina so it can readily dilate when labor commences. It also relaxes the ligaments and joints in the pelvis which furthers aids the mother during the delivery process so the infant can drop into the pelvis carriage in preparation for birth.

The Final Stage of the Labor Process

During the final stage of pregnancy, relaxin promotes the rupture of the placental membrane surrounding the infant when labor begins. It also elongates the cervix for dilation.

Unpleasant Aspects of Relaxin


The hormone basically relaxes many areas of the mother’s body and is often the primary culprit causing heartburn during the advancing pregnancy. The growing baby pushes upwards against the stomach. Heartburn occurs because of the combination of the uterus pushing into the stomach and the elevated levels of relaxin which further relaxes the gastrointestinal system causing the stomach acids to flow up into the esophagus.

Wobbly Walk

During the final trimester, relaxin helps relax the body’s pelvic joints and lower spine so the baby can more easily be expelled from the birth canal. The loose joints often cause the pregnant woman to suffer from a wobbly gait that is less than graceful. Extra care should always be taken when walking during the final trimester because of the new balance issues and looser joint ligaments.

Additional Body Aches

Although the hormone is geared towards relaxing the ligaments and joints of the pelvis in anticipation of the birth, it also loosens all ligaments in the joints of the body. This can cause ongoing backaches, neck pain, knee discomfort, ankle problems, and shoulder issues. However, after the birth, the pains quickly dissipate.

Dangers of Excessive Relaxin

High levels of the hormone have been associated with premature birth. It is believed that the excessively elevated levels of the hormone in a mother’s system may contribute to the early rupture of the membranes and the softening of the cervix which causes miscarriage, according to the American Journal of Obstetrics and Gynecology.

What the Experts Say About Relaxin

“Relaxin appears to be involved in many important areas of human reproduction and the long-term therapeutic potential in understanding its potential roles in the human is exciting.”

Dr. Alastair H. MacLennan, University of Adelaide in South Australia.

“We don’t really know how long relaxin stays active in the postpartum body. The implications of this apply to every joint in the body. Pregnant and postpartum women should take special care to be gentle on their joints.”

-Peg Mass, Swedish Medical Center

“This study, which mainly targeted small renal arteries isolated from rats, is consistent with our earlier work showing that relaxin increases renal blood flow and kidney filtration by as much as 40 percent in non-pregnant rats. If we can discover the hormones responsible for these pregnancy changes and how they work, they might be useful for fighting high blood pressure and other cardiovascular diseases such as heart attack and stroke.”

-Kirk P. Conrad, M.D., Magee-Womens Research Institute

“Relaxin is an ovarian hormone that was discovered in 1926. When a woman becomes pregnant, relaxin rapidly increases in the blood. Changes in the renal circulation are nearly at their highest by the end of the first trimester of pregnancy.”

Jacqueline Novak, Ph.D., University of Pittsburgh School of Medicine.

There is very little doubt that relaxin promotes not only a healthy pregnancy but an easier birth. The body naturally produces reliable levels of the hormone to ensure that your body meets the needs of your growing baby and helps your body transition easier for the birthing process.

Kimberly Sharpe

Based in Florida, Kimberly Sharpe has been a full-time writer since 2006. Her writing has a strong focus on travel, parenting, outdoor sports, gardening, health issues, pets (both domestic and exotic), home improvement, DIY, and business promos. Her work has appeared in USA Today, MORR Gear, Hipmunk, Travelocity, Livestrong, Hotels.com, Hydro Live, Maximum Yield, eHow, Yahoo News, SF Gate, Garden Guides, Whitefence, S.F. Gate, fixr.com, and numerous other publications. She has traveled extensively throughout Europe, India, and Sri Lanka in an effort to expand her knowledge and enhance her writing skills.

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