Modelling the Uterus

There’s nothing wrong with using a model to reimagine some function of the body, models are the scientists’ equivalent of metaphor (McGilchrist, 2022). You have to start somewhere, and if, like me, you question the traditional model of uterine function (the three Ps model – Powers, Passenger and Passage) you have to replace it with something else. Why do I think a balloon is a good model of the uterus? It so happens that the good old fashioned party balloon is ideal. It’s the right shape. It expands and contracts, it is elastic, it has a knot to keep its contents in, it can be reused – so long as it doesn’t pop. And, best of all, it is a celebration of birth! (I first wrote about this in 1997 in Midwifery Matters and will add it to the resources page when I have permission.)

I have elaborated and expanded the model since 1997. The orange balloons at the top of this page are based on that elaboration, look carefully and you can see a network fading out and the balloon shrinking. It’s not a perfect model; the uterus grows as well as stretches during pregnancy and the muscle of the uterus, the myometrium, isn’t composed of just one substance but of at least three: muscle, collagen and elastin. The uterus gains substance during pregnancy and loses substance after birth. (We could work out what it loses by analysing the lochia which is hardly ever considered by scientists, I suspect because it is a waste product.) Also, the knot of the uterus, the cervix, is not a true knot but composed of grossly stiffened material – as if you had dipped the end of the balloon in varnish – it won’t stretch until the baby is ready to be born and wants to make his way out. Only then does it start to lose its stiffening. And we have a way of assessing this – the Bishop’s score.

The biological substance of the uterus has much in common with the biological stuff of the cervix; only the proportions of each material are different. The cervix has far less muscle (10-15%) than the main body of the uterus and much more collagen. In fact the proportions of muscle and other tissue also vary within the uterus itself – there’s more muscle at the fundus than the lower segment. The uterus is composed of three main materials. For two of these substances, the clues are in the names, muscle = power, elastin = elasticity. The third, collagen, acts as stiffening, or scaffolding to provide a framework for the muscle. We know that the substance of the cervix changes before labour, but I maintain that so also does the substance of the uterus. Both become less rigid, more stretchy, a bit like the bulb of a turkey baster morphing into a party balloon.

I want to talk more about the substance of the working component of the uterus, the myometrium (myo = muscle; metrium = womb (latin)). The myometrium is made of smooth or visceral muscle. It is the most powerful muscle in the body – it has the power to push out a 8lb (or more) baby all by itself. Babies can be and are born when their mother’s lower body is paralysed by an epidural; even mothers in a coma can give birth. So, yes, the textbooks got it right in referring to as ‘The Powers’. BUT muscle is not the only component; the myometrium is what materials scientists call a composite material.

Muscle cells are too sloppy and formless to exist without some sort of scaffolding to support them. And this scaffolding has been almost totally overlooked – to the extent that scientists studying the physiology of the uterus by dissection used to cut most of it away so that they could analyse individual muscle cells. And in so doing I believe that they threw away the very substance that keeps the uterus relatively quiet during pregnancy. (On the contrary, the significance of collagen is acknowledged at the cervix, acknowledged in the very existence of the Bishop score.)

Other researchers have wondered about the role of collagen. Muscle cells are embedded in the extra cellular matrix, a network of scaffolding largely made of collagen. This scaffolding stiffens the uterus, making it less stretchy. There are whole layers of organisation, muscle bundles surrounded by collagen, grouped into larger sections, perhaps also with pacemaker type cells (interstitial Cajal like cells) interspersed. Fifty years ago, Thomas Morrione analysed slivers of uterine tissue ‘harvested’ at caesarean section, there was more collagen in samples taken after elective caesarean than at in-labour caesarean (well before the days of wholesale induction, these women were ready to labour). In other words, there was a loss of collagen at the site of the incision when labour had started spontaneously. The loss of some of this collagen scaffolding would make the uterus more stretchy and, since stretch leads to contraction, a labour contraction could now spread throughout the uterus, eventually enabling it to contract all at once as a whole, as it does in labour. In other words, progress in labour depends on how stretchy the uterus becomes, how far a contraction can travel beyond its place of origin – the pacemaker site.

Labour is now known to be an inflammatory process involving prostaglandins – the inflammation leads to tissue loss, and I believe that the lost tissue is collagen – the very scaffolding that is preventing widespread stretch. Textbooks will inform you that uterine muscle is the only muscle that reverts to less than its original length after contracting, loss of collagen may well be the explanation.

The first stage of labour is a positive feedforward process led by what I call the stretch:contract reflex. We are not aware of gut and colon contractions moving food and waste products through the body but, starting with the Braxton Hicks contractions of pregnancy, we do become aware of uterine contractions. As they become stronger they are perceived – and labelled as – painful. After all, they have a lot more work to do – it’s not called labour for nothing!

So I have come to the conclusion that the first stage of labour is a whole series of stretch:contract reflexes that move the baby down towards the cervix and allow the cervix to open once its own collagen has ‘melted away’.

The stretch:contract reflex is overlooked in birth, scientists have become obsessed with hormones and biochemicals (as indeed I was when I first embarked on my journey towards understanding birth). And what does the stretching? The baby of course. The baby is no passive passenger but actively seeks the exit – once the substance of the cervix and uterus have changed enough to enable him to do so. The purpose of the first stage of labour is to position the baby in the best position for the journey through the pelvis. Yes, the cervix must open but this can be thought of as an inevitable consequence of untying the cervical knot to reveal the exit to the baby, and of the change in the substance of the cervix and the uterus. The cervix opening is the inevitable result of internal pressure from the baby and the waters. The cervix opens because now it can. Second stage is a new part of the story for another post.


McGilchrist I (2022). The Matter with Things volume I p494
Morrione TG and Seifter S (1962). Alteration in the collagen content of the human uterus during pregnancy and post partum involution Journal of Experimental Medicine,115(2): 357–365. doi: 10.1084/jem.115.2.357