When we think about the primary hormones of pregnancy, we might think about progesterone and oestrogen. We might have heard about hCG and its association with morning sickness and we might even know a little about the role of oxytocin in labor and birth.

Relaxin hormone does not automatically spring to mind. If it does, we might only be aware of its negative effects on pelvic pain. In fact the relaxin hormone has a very important effect on our whole reproductive system; it plays much more of a leading role in conception, pregnancy and birth than we might realize.

What is relaxin hormone?

Relaxin is a reproductive hormone produced in the reproductive tract of all mammals; it belongs to the same family of hormones as insulin. It was first discovered and named in 1926 by Dr Frederick Hisaw, during his research on guinea pigs.

Interest in relaxin research has increased and waned over the decades and has given rise to challenges for the progress of further research. Although more research is still needed into this particular hormone, a number of discoveries have been made.

What is the relaxin hormone responsible for?

Relaxin hormone is best known for its effects on the female reproductive system and, specifically, for the significant role it plays in the menstrual cycle.

Relaxin does have a role outside of menstruation, however; it has effects on other organs and areas of the body, in both males and females.

In men, relaxin hormone is secreted from the prostate gland. It can be detected in semen but is not usually found in the general circulation. Its role in male reproduction is not fully understood. It is thought to play a role in increasing movement in the sperm cells but this has not been well researched.

Relaxin’s role during your menstrual cycle

The relaxin hormone is produced by the corpus luteum, which forms on the ovary following ovulation. The levels of relaxin hormone rise, post-ovulation, thought to be stimulated by luteinizing hormone (LH). If conception does not occur during a menstrual cycle, relaxin hormone levels drop and remain low until the next cycle.

If conception does occur, relaxin hormone production will remain high throughout early pregnancy, levels are at their highest, during the first trimester. Following conception, the relaxin hormone serves to calm the uterus by inhibiting contractions and minimizing the odds of miscarriage.

Relaxin during early pregnancy

Only when the embryo implants into the wall of the uterus does a pregnancy technically begin. The relaxin hormone levels rise to support the ongoing pregnancy by preparing the uterus for implantation of the embryo into the uterine wall. This process is critical for the success of the pregnancy. If the uterine lining is not adequately primed in preparation for the embryo’s arrival, adverse pregnancy outcomes are inevitable, as a pregnancy cannot survive outside the uterus.

You can learn more about what happens when an embryo implants outside the uterus by reading our article Ectopic Pregnancy | Symptoms, Signs And Treatment.

Relaxin hormone also helps the body prepare for pregnancy by supporting the process of decidualisation, which is the term for the dramatic changes that happen to the lining of the uterus to make it a welcoming environment for the embryo to embed itself into. The lining needs to remain thick and to develop a rich blood supply through development of new blood vessels, to support and nourish the new little life.

These processes are complex but successful implantation is critical for the survival of the pregnancy. In human beings, there is a high ‘fail rate’. Research suggests that only 50% of all conceptions will advance beyond 20 weeks gestation and, of all unsuccessful pregnancies, 75% are due to unsuccessful implantation.

Side effects of relaxin in early pregnancy

In early pregnancy, high levels of relaxin, as well as other hormones, such as progesterone, are responsible for some of the unpleasant gastrointestinal problems, such as constipation, that pregnant women might experience.

The hormone relaxes smooth muscle (the muscles we can’t control) in the body, such as the muscles needed to push food through the bowel. As a result, it takes longer for food to pass through the digestive tract, so that the body can absorb as many nutrients as possible for the baby. The downside, however, is it can leave us feeling constipated.

During pregnancy, relaxin is secreted by the placenta, the lining of the uterus and the membranes surrounding the baby.

Related reading: Is Constipation A Sign Of Early Pregnancy?

Relaxin during later pregnancy

Although levels are highest during the first trimester, relaxin also has important effects in the second half of pregnancy.

As its name suggests, relaxin has a relaxing effect on the muscles and ligaments of the musculoskeletal system. This not only helps our bodies accommodate for the rapid growth of a baby, but the effects of relaxin also specifically work on the pelvis, in preparation for childbirth.

Pelvic pain

Relaxin hormone helps to relax the ligaments and joints in the pelvis. It’s important for birth that the pelvis remains a dynamic structure – not entirely fixed in one position. As the ligaments stretch gently, extra space is created in the pelvis for the baby to pass through. This enables the baby to get into the optimal position while descending through the birth canal. It also increases elasticity in the pelvic floor muscles to enable the baby to be born more easily.

Related reading: Small Pelvis Big Baby | 3 Truths About CPD.

Relaxin helps our blood vessels to relax to accommodate a 30-50% increase over our normal blood volume and blood flow. This supports the baby’s nutritional demands and prevents our blood pressure from spiking.

Too much relaxin?

Because of higher relaxin hormone levels, pregnant women might need to be extra careful, to avoid overstretching when bending, lifting or exercising as it’s far easier to pull or overstretch muscles and cause pain.

High levels of relaxin hormone can also lead to pelvic girdle pain (PGP), which can be uncomfortable for many women, and extreme and difficult to control in some cases. Movement in the pelvis caused by the stretching ligaments, plus the extra weight and changes to posture during pregnancy, are the cause of this pain. If you experience PGP, although it might be frustrating, just remember that your body is preparing your pelvis for childbirth, to make the process as efficient as possible.

Towards the end of pregnancy, relaxin promotes growth of the placenta, the rupture of membranes (ROM) and the softening and effacing of the cervix prior to birth. Further research is needed, but it is believed that too much relaxin could be linked to an increased risk of premature birth, due to its association with ROM and opening the cervix.

Related reading: Preterm Birth | Ways To Prevent Preterm Birth.

Relaxin during breastfeeding and postpartum

Although relaxin levels decrease after birth, they will remain elevated for a number of months post birth. It is estimated to take about 5-12 months before levels of circulating relaxin return to normal. The hormone might be present for longer in mothers who are breastfeeding.

Some researchers also believe that relaxin has an effect on growth and development of the milk producing tissue in the breasts and that it is an important component of breast milk.

It’s important to be aware that you might still feel the effects of relaxin in your body, postnatally. If you’re planning to return to exercise in the months after giving birth, you’ll need to consider appropriate exercises, as the risk of injury is greater during this time.

Separation of the abdominal muscles is common during pregnancy and postpartum, and is known as diastasis recti. This condition loosens and weakens the muscles of the abdomen and also occurs as a result of relaxin.