This mysterious hormone gets the blame for many pregnancy and post partum complaints, but is it really the culprit? Here’s what the research tells us about relaxin and how it affects the body in pregnancy and the early weeks post partum.


Does relaxin cause pelvic girdle pain in pregnancy?

Relaxin’s best known job is the role it plays in changing our connective tissue, most notably the ligaments which provide stability in the pelvic joints. Research has shown that relaxin causes the ligaments to become more stretchy so that the pelvis can open to allow the baby to move through the birth canal and be born. Relaxin doesn’t just seem to affect the pelvic joints – research has shown that it also causes the TMC joint (the bottom joint of your thumb) to become more unstable.

What’s interesting about this is that this instability did not correlate with pain. That’s right – An unstable joint doesn’t necessarily cause pain. Researchers have found is that relaxin does cause increased laxity in some joints.However, study participants who had joint laxity and higher levels of relaxin in pregnancy weren’t any more likely to complain of pain than the participants who had lower relaxin levels and more stable joints.  So no, we can’t say that relaxin causes pelvic girdle pain during pregnancy.

In fact, in one fascinating study, researchers found that there may be an association between gestational diabetes and pregnancy pelvic girdle pain. Research continues!



What about people who have joint hypermobility?


Joint hypermobility is a (usually) benign condition where your joints are more ‘stretchy’ than most people’s. Some people can have hypermobility at all of the joints, while many people have one or two hypermobile joints. Studies haven’t found a definitive link between hypermobility and relaxin, but we do know that people with hypermobility tend to adopt unusual postures. This can, in itself, cause pain. There isn’t any evidence that increased levels of relaxin due to pregnancy contribute to pain in people who have hypermobile joints.


Are my relaxin levels higher if I’m breastfeeding? Could they be causing my pain?


A lot of the research into relaxin’s role in lactation is on animals and therefore can’t really be extrapolated straight onto humans. It is thought that during pregnancy relaxin helps to prepare the breast to feed your baby.

 Studies in the 1950s seemed to indicate that relaxin ‘disappeared from the blood within 24 hours post partum’.  This research is REALLY old (I mean for research; apologies to anyone who was born in the 50s!). However, there is no evidence that relaxin levels are higher in breastfeeding mothers or that they are a factor in pain in the post partum period.


So what is causing my pain?


There are many factors that can contribute to joint and muscle pain in pregnancy and post partum. For more information on pregnancy pelvic girdle pain, have a read of two blog posts I did on PPGP here. As for post partum pain, here are some factors to consider before you start worrying that relaxin and breastfeeding might be the problem.




All our hormones go through huge changes post partum. Because our hormones work together constantly to balance and complement each other, the balance of ALL our hormones, not just relaxin or oestrogen or thyroid hormone alone, is what’s important. We know that, among other functions, thyroid hormone is involved in healthy muscle function. Higher levels of oestrogen in proportion to progesterone can cause increased sensitivity to pain. We also know that around 5% of women can develop a thyroid hormone imbalance after pregnancy.




Probably the biggest shock after the birth of a baby for many mums, is the fact that babies do not sleep. At least never as much as we want them to. Breastfed babies can tend to wake often at night to feed or just to reassure themselves that ‘yes, you’re still awake!’ Not getting the sleep you need increases cortisol (our body’s stress hormone), which can increase pain sensitivity.

Add to that the fact that your body uses the time that you’re asleep to ‘reset’ and heal your muscles, joints and body systems in general after the stresses of the day and you’ll begin to understand why you’re sore, forgetful and ready for bed at 6pm!




Are you eating healthy meals fairly regularly? Many new mums find that they’re so busy they can forget to eat! Sometimes general aches and pains or just not feeling great can be our bodies reminding us that we need to take better care of ourselves.

Forgetting to eat or grabbing something quick and convenient but maybe not very nourishing, can leave us short of important vitamins, minerals and other nutrients that our muscles and joints need to function well and without pain.


Pregnancy-related postural changes


You know that weird feeling when you first get up after birthing your baby? There’s still a bit of a bump there but you feel so much lighter. Your centre of gravity has just shifted waaaay back from where it was the day before! Takes a bit of getting used to, doesn’t it? Your muscles and joints now all have to change the way they’ve been working over the past 9 months to accommodate this sudden change. Some muscles will need to work harder. Some will need to gradually loosen out. And of course some will need to work faster and harder, because, let’s face it, you will be working harder now that you’ve got a new baby.

In between all those periods of activity, you may be sitting (or lying) for long periods for feeds, whether you’re breastfeeding or formula feeding. It can be helpful to be mindful of your habits when you sit or lie. Do you always lie on the same side? Do you sit with the legs crossed or folded up beside you to one side (usually the same side)?

Do you make sure to have support for your arms? This ensures you’re not having to support the weight of your baby for sustained periods. Doing these things ‘the odd time’ is ok. It’s when they become a habit that we see muscles beginning to become tight and sore. It can also contribute to joint and tendon problems.



Squeeze your shoulders up to your ears. Tighten your jaw. Squeeze your tummy muscles in. How does your body feel? Stress increases tension in the muscles. For many of us, the spots where we tend to hold most tension are the jaw, shoulders and abdominals. Stress can also cause the release of a hormone called cortisol which causes the fight or flight response. This raises blood pressure. It increases our breathing rate and directs blood away from the brain to the arms, legs, heart and lungs.

You may find that you have difficulty thinking straight, feel tense, your mouth is dry or you may get headaches. You may find it difficult to get to sleep even when your baby is asleep. Achey muscles and joints can be a result of chronic stress.

Yep, it’s your body reminding you again that you need to make time for yourself – maybe asking someone to mind your baby for a while so you can catch up on sleep. Get out for a walk with your baby in the buggy or sling. Make time for a date night with your partner or listening to a relaxing body scan – whatever helps you to unwind and ‘reset’.


So does relaxin cause or contribute to post partum joint or muscle pain?

There’s certainly no evidence that it does. In fact, there is no definitive evidence that higher levels of relaxin are in any way associated with higher levels of pain. Post partum joint pain is most likely caused by a combination of many factors including those listed above. And because of this, as a women’s health physio I would NEVER advise a woman to stop breastfeeding. Some women will choose to stop breastfeeding because of post partum pain and that’s ok – once her decision is properly informed. To me, that means providing evidence based information and advice and non-judgemental and compassionate support.

If you are suffering from post partum muscle and/or joint pain, your local women’s health physiotherapist should be able to help you. These are specialist professionals who take a holistic view of the woman and can advise you based on a thorough and detailed examination. You can find a Chartered Physiotherapist in Women’s Health on or email me at


For more information on pregnancy pelvic girdle pain see my blog posts:


Pregnancy Pelvic Girdle Pain – When Pregnancy is a Pain in the Bum


Tips to Help Kick Pregnancy Girdle Pain


Exercises for Pregnancy Pelvic Girdle Pain





Circulating levels of relaxin are normal in pregnant women with pelvic pain – Albert et al 1997


Characterization of the relationship between joint laxity and maternal hormones in pregnancy – Marnach et al 2003


The Relationship Between Serum Relaxin and Knee Joint Laxity in Female Athletes – Arnold et al 2002


Pregnancy-related pelvic girdle pain and its relationship with relaxin levels during pregnancy: a systematic review – Aldabe 2012


Joint laxity and the benign joint hypermobility syndrome in student and professional ballet dancers – McCormack et al 2004


Hip, knee, and foot pain during pregnancy and the postpartum period (article) – Vullo et al 1996


Mice without a functional relaxin gene are unable to deliver milk to their pups 1 – Zhao et al 1999


The Effect of Reproductive Events and Alterations of Sex Hormone Levels on the Symptoms of Fibromyalgia – Østensen et al 2009


Hormonal and Reproductive Factors are Associated With Chronic Low Back Pain and Chronic Upper Extremity Pain in Women: The MORGEN Study – Wijnhoven et al 2006

Diabetes mellitus and pelvic girdle syndrome in pregnancy – Is there an association? – Eberhard-Gran et al 2009


Sex hormones, central nervous system and pain – Aloisi et al 2006