Relaxin hormone


Relaxin is a hormone, belonging to the same family of hormones as insulin, which is present in both men and women. In women, it is secreted into the circulation from the ovaries and breasts. During pregnancy it is also released,

  • from the placenta, an organ that acts as a connection between the fetus and the mother;
  • from the basal decidua: decidua, is the uterine mucosa modified by pregnancy which in the last phase of childbirth is eliminated in the expulsion of the placenta and all its annexes; basal, is the portion of the decidua that interacts with the trophoblast.

In the pregnant woman, relaxin plays a dual role.

  1. At the beginning of pregnancy,  at the time of implantation  , together with progesterone, it prepares  the maternal tissues to welcome the embryo.
  2. At the end of the pregnancy , in preparation for the birth, it produces the shortening / cervical smoothing (ie the thinning of the cervix) and the dilatation of the neck of the uterus of the pregnant woman.

Although the function of peptide hormones of the relaxin family is unclear, several   relaxin-like peptides have been discovered in the last decade.

The activity of relaxin in the female reproductive cycle and in pregnancy

During the female reproductive cycle and during pregnancy, it performs its functions as follows:

After ovulation, in the second half of the menstrual cycle.

  • Its levels increase.
  • (At this stage) relaxin is thought to relax the lining of the uterus, inhibiting contractions, and prepare the lining of the uterus for pregnancy.
  • After this period and in the event of a missed pregnancy, relaxin levels will drop and return to normal again.

In case of an ongoing pregnancy.

  • In early pregnancy : Relaxin reaches its maximum level during the  first trimester . It is believed that during this period it promotes the implantation of the developing fetus in the wall of the uterus, and the growth of the placenta. In early pregnancy relaxin also inhibits contractions in the wall of the uterus to prevent premature birth.
  • During pregnancy, it can regulate the mother’s cardiovascular and renal systems so that they adapt to the increased demand for oxygen and nutrients for the fetus and to process the resulting waste products.
  • Towards the end of pregnancy,  relaxin helps rupture the membranes surrounding the fetus, and the opening and softening of the cervix and vagina to help the delivery process.
  • Close to birth.  There is some evidence to support the hypothesis that relaxin may relax the ligaments in the anterior part of the pelvis, again to facilitate childbirth.

Its effects on other systems of the human body

In addition to those seen above, the effects of relaxin on other systems and organs of the human body are as follows:

  • Relaxin reduces tissue fibrosis in the kidneys, heart, lungs and liver and promotes wound healing. Tissue fibrosis is the formation of hard tissue as a result of inflammation, which can lead to scarring and loss of organ function. This fact opens up possible future scenarios in which relaxin could (but has yet to be proven) be useful in treating heart failure.
  • Relaxin can affect our blood pressure by relaxing blood vessels.
  • It can promote the growth of new blood vessels.
  • It has an anti-inflammatory effect.
  • It acts on the metabolism of collagen by inhibiting its synthesis and improving its breakdown by increasing the matrix metalloproteinases MMPs (a family of enzymes).
  • It is a potent renal vasodilator.
    For these numerous effects, the therapeutic perspectives of relaxin certainly seem to be numerous.

How relaxin is released in the body

The mechanism by which relaxin is released in the body is not fully understood. It is believed,

  • that during the menstrual cycle, the relaxin production from the ovaries is stimulated by the luteinizing hormone and the pituitary gland,
  • that during pregnancy, its production is also stimulated by  chorionic gonadotropin .

It is unclear whether relaxin may also send feedback to the pituitary or fetus to influence luteinizing hormone or human chorionic gonadotropin levels, and thereby control its release.

Its levels, too high or too low

Pathologies related to a deficit or an excess of relaxin have not yet been described, although some studies seem to suggest that,

  • high levels of relaxin in the pregnant woman  are associated with premature births, presumably due to the effects of this hormone on the rupture of the fetal membranes and on the opening of the cervix: however, further studies are needed to confirm this hypothesis;
  • low levels of relaxin  can contribute to the genesis of scleroderma (a “hard skin”) and fibromyalgia (a chronic disease with widespread pain, asthenia, muscle stiffness).

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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