Relaxin - the wobbly hormone

Have you heard about a hormone called relaxin? Relaxin is a hormone produced by the ovaries and the placenta that is secreted into the bloodstream. Among other things, it has important effects in reproduction, pregnancy and childbirth.

Conception

When you’re not pregnant, you still have relaxin in your body. Relatively low levels rise after ovulation and fall again if you do not get pregnant. BUT relaxin has beneficial effects upon the endometrium which are responsible for establishment of pregnancy. At a biochemical level, it stimulates structural changes in the endometrial cells and the surrounding tissue. It increases the concentration of local immune cells which are required for implantation. This basically means that it prepares the lining of the uterus for pregnancy. That was too sciencey – but it helps make a baby – after the sex part.

Once you’re pregnant, in preparation for growing a baby, relaxin relaxes the connective tissue to allow your body to actually fit a growing baby. Leading up to birth, relaxin also “ripens” or softens and widens the cervix.

First Trimester

During pregnancy, your relaxin levels peak in the first trimester. It is thought to be helping develop the baby and the placenta, plus prevent contractions so that you don’t go into early labour. Relaxin assists the cardiovascular and renal systems (kidneys) to help adapt to the increased demand for oxygen and nutrients for the mother and baby. It is thought to do this by relaxing the walls of the mother’s blood vessels, causing vasodilation (widening of the blood vessels) to increase blood flow to the placenta and kidneys[1].  

Relaxin is known to “remodel” pelvic connective tissue, but there is no proven correlation between pelvic girdle pain and levels of relaxin.[2] Additionally, peripheral joint laxity increases as pregnancy progresses, but there is no correlation with relaxin in these joints[3]. This suggests that although there may be joint laxity (a slightly looser joint), relaxin is not actually the cause of joint pain such as pelvic instability[4].

Third Trimester

Towards the end of your pregnancy relaxin increases again, but this time not as high as the first trimester[5]. At this point its role is to soften the cervix and to help rupture the membranes; “break your waters”.  There is some evidence that relaxin can relax the ligaments at the front of the pelvis at birth to ease delivery of the baby. Studies have shown that if you are having twins, the level of relaxin rises even more.

Forth Trimester

Research has shown that by the time your baby is only 3 days old, relaxin levels fall to almost non-pregnant levels.

Well, that’s a lot of great science, but why do I need to know this?

1) Relaxin is in your bloodstream, not just your pelvis. So, lots of things in your body will be affected.

2) Connective tissue (that is being softened by relaxin) is holding up your digestive system, bladder and bowels. So, with a huge increase of relaxin in the first trimester, your gut will take a hit. The concoction of hormones circulating can make you feel nauseas, have diarrhoea or constipation. It is comforting to know that the levels of relaxin start to plateau after the first trimester.

3) Vasodilation (mentioned above) can reduce blood pressure. This means that you might be at risk of feeling a bit lightheaded if you stand up too quickly after lying down. Generally, this isn’t a cause for great concern, but be careful when you get up.

4) Increased joint laxity needs increased neuromuscular control. Exercising when your tendons and ligaments have increased laxity, can put you at greater risk of injury, if your body is not used to this. It makes sense that you should avoid “over-stretching” and choose exercise that you can control movement. So, if you’re experiencing an uncomplicated pregnancy, you should be encouraged to continue, or to initiate safe physical activities, with appropriate body control. 

Fun fact: Men also produce relaxin from their prostate gland and it can be found in semen, but it is not circulated into the bloodstream.

[1] MacLennan AH, Nicolson R, Green RC. Serum relaxin in pregnancy. Lancet. 1986 Aug 2;2(8501):241-3. doi: 10.1016/s0140-6736(86)92068-4. PMID: 2874276.

[2] Aldabe D, Ribeiro DC, Milosavljevic S, Dawn Bussey M. Pregnancy-related pelvic girdle pain and its relationship with relaxin levels during pregnancy: a systematic review. Eur Spine J. 2012 Sep;21(9):1769-76. doi: 10.1007/s00586-012-2162-x. Epub 2012 Feb 4. PMID: 22310881; PMCID: PMC3459115.

[3] Schauberger CW, Rooney BL, Goldsmith L, Shenton D, Silva PD, Schaper A. Peripheral joint laxity increases in pregnancy but does not correlate with serum relaxin levels. Am J Obstet Gynecol. 1996 Feb;174(2):667-71. doi: 10.1016/s0002-9378(96)70447-7. PMID: 8623804.

[4] Petersen LK, Hvidman L, Uldbjerg N. Normal serum relaxin in women with disabling pelvic pain during pregnancy. Gynecol Obstet Invest. 1994;38(1):21-3. doi: 10.1159/000292438. PMID: 7959320.

[5] Szlachter BN, Quagliarello J, Jewelewicz R, Osathanondh R, Spellacy WN, Weiss G. Relaxin in normal and pathogenic pregnancies. Obstet Gynecol. 1982 Feb;59(2):167-70. PMID: 7078861.