The Reactive Uterus or the Birth Day Balloon


IT'S ARM's 21st birthday and so this seems an appropriate time to air another theory that has escaped the textbooks. Before writing a book intended to be mainly about home birth, I thought I really ought to learn about how the uterus works in labour. To my surprise there was very little information. Learned tome after learned tome outlined the three stages of labour telling the reader what happens but nowhere did it say how.

Anatomy and physiology sections were full of information about structure and there were snippets of useful information such as the standard description of longitudinal muscle contracting and retracting to pull up the cervix, but the uterus was never seen as a coherent whole. What is responsible for contractions coming in waves? Why do contractions get stronger and closer together during labour? I was left with the impression that for the professionals, just like women in labour, contractions just `happen'. They knew what contractions did, but they didn't know how and why. Most textbooks deal only with treatment for `inefficient/malfunctioning/lazy' uteri, and that treatment is based on pragmatism rather than science, which could be described as the sledgehammer approach. Perhaps if we knew more about the natural function and what disrupts it we would need less recourse to unphysiological amounts of oxytocin, to forceps and to the scalpel.

I think that if we know what is happening, how and why, then we can accept contractions gladly and cope with labour better. People caring for labouring women will have a better idea what is likely to impair uterine function and what will facilitate it.

I had to delve deeper and deeper into the physiology literature to discover how the uterus works in labour and I ended up with a radically different view of the labouring uterus. I learned that it should not be looked at as an isolated organ, but as a dynamic and reactive part of a triad consisting of the mother's mind, her whole body and the physical activity of her baby. All three work together as a united whole to allow the baby to be eased gently into the world.

Before looking more closely at these three effects upon the uterus in labour, it would be helpful to know more about its mode of action. And I shall start by changing your thinking about it and making it more `user friendly'.

Let's forget the anatomy and physiology for the moment and look at the mechanical properties of the muscle of the uterus. Physics may not be your strong subject (to tell you the truth it wasn't mine either) but fortunately that doesn't matter because there is a simple illustration that anyone could understand:

The Womb's a Balloon

- It's a hollow elastic bag

- It's about the same size

- It's flat with touching sides when empty and pear-shaped when `blown up' (gravid)

- If it is stretched it will revert to its original shape - it has elastic memory. (Imagine putting your fist against a blown-up balloon, then withdrawing it, the balloon regains its pear shape - you do this from the outside but the baby will do it from the inside.) The uterus is more dynamic than a balloon; during labour myometrial cells react to being stretched by becoming even smaller - contraction leads to retraction.

- It can expand to many times its original size and then revert to near its previous size; deflated balloons never quite retain their virgin state, neither does the uterus.

- It can be under enormous pressure and yet not burst

- It can be tied shut (quite literally with a Sirodhka stitch)

- Up to a certain point it can cope with slight flaws in its substance, e.g. scar tissue from a caesarean section - but you can never quite tell

- It is fragile yet immensely strong

- If maltreated it can burst and last but not least...

- Something usually used to celebrate life can lead to tears or tears

The womb is not just a passive elastic bag to be manipulated from the outside, it can generate more of itself; it grows from 2 oz to 2 lb during pregnancy and then within six weeks of birth it digests itself to go back to its pregravid self (autolysis). It is not an inert, passive elastic bag floating aimlessly in the air but it is dynamic and actively responds to many and various forms of stimulation.

The Dynamic Cervix

Like the main body of the uterus, the cervix is also made of elastic smooth muscle, but it has an added matrix of collagen and connective tissue which is generated upon instructions from hormones. This makes it stretch resistent during pregnancy so that any shortening longitudinal muscle cannot pull the cervix out and up. You could think of this as pregnancy applied superglue which must be dissolved before labour can begin in earnest. With the superglue in position the cervix cannot stretch. Even when there is a discernable opening, as there can be toward the end of pregnancy, the uterus will not `empty itself'. As a last precaution against precipitate labour, cervical muscle contracts to keep itself closed - it can contract even in the early stages of spontaneous labour, although is more likely to contract during induced labour (Olah, Gee and Brown, 1993).

In purely mechanical terms, if you untie the knot of a balloon the mismatch of pressure inside and out forces the air out through the neck. If the uterus and the cervix are ready to labour, `untying' the cervix will mean that labour may happen automatically. The body will automatically attempt to equalise the pressures either side of the cervix.

The contractions of labour result from a mechanical reflex action but labour is not nearly so quick as deflating a balloon, it needs to be well controlled to protect both the mother and her child; there are built-in constraints, the body must control labour so that the baby makes a gentle entrance into the world. Precipitate labour can be very dangerous, ejecting the baby too fast and tearing the mother in the process.

The active uterus

The uterus is a dynamic, active organ, not just a baby bag. The usual state of the uterus is to be active. The myometrium is made of plain, smooth or visceral muscle and this type of muscle is active all the time. Until cells actually die, smooth muscle including strips of myometrium - is active in the test tube. Smooth muscle enables reflex body actions to go on all the time whether we like it or not - lungs, blood vessels, stomach, these all have to carry on pumping substances around the body when and where they are needed to supply nutrients to sustain life and to expel waste products. Reproductive smooth muscle has an equally important role, it carries life forward to the next generation.

The activity of the uterus is achieved by five mechanisms: stretch, neural, electrical, hormonal and metabolic.


Uterine muscle, like other smooth muscle, reacts to being stretched by contracting. This could be called the stretch-contract reflex. The uterus also has this stretch-contract reflex action, but it is modulated to a greater or lesser extent throughout the menstrual cycle depending on whether contraction-promoting hormones or contraction-inhibiting hormones predominate (does the oestrogen/progesterone ratio ring any bells?). In his ejaculate, the male provides prostaglandins to make the uterus more volatile to help sperm on their journey to the fallopian tubes.


Smooth muscle is responsible for maintaining essential life functions and is under the control of the autonomic branch of the parasympathetic nervous system; it cannot be switched on or off by an act of will. However, paralysed women can labour just as well as their intact sisters - perhaps even better since nervous messages seem only to damp down uterine activity, never to improve it.

Electrical activity

Electrical connections between uterine muscle cells can determine whether or not something happening in one area of the uterus can affect adjacent areas - whether contractions can spread or not. The more connections there are, the further a contraction will spread. Sometimes a previous scar will stop the spread of contractions because cells either side of the scar cannot influence each other; this makes labour incoordinate and inefficent (Mary Cronk, personal communication, 1997).


Effects of increased stress hormone secretion are readily apparent in smooth muscle organs, we can get stress-induced diarrhoea or constipation, we have `gut feelings'. Our blood pressure goes up under stress, The uterus also mirrors our mental state. (This is the direct opposite of hysteria where the uterus was supposed to affect our mental state - the mind affects the uterus, not vice versa.)

Oestrogen - spreads contractions; allows a uterus to be active throughout its entirety by making electrical links between one myometrial cell and the next

Progesterone and beta-endorphin - dampen down the usual reflex action of uterine tissue to react to stretch by contracting

Oxytocin - initiates contractions by freeing prostaglandins

Prostaglandins - dissolve the cervical `superglue'; released locally as a response to oxytocin to activate myometrial cells which then activate others...

Adrenaline - stops contractions

Noradrenaline - allows contractions


The only explanation for the wave-like arrival of contractions and the sensation of pain that I have found is in a paper by Susan Wray, a physiologist and a mother of three (Wray, 1993). Muscle cells need oxygen to contract but in contracting they cut off their oxygen supply and become ischaemic and so stop contracting, once they relax their blood and oxygen supply is restored and so they can contract again. Ischaemia is associated with pain, the pain of cramp, the tissue is shouting for its oxygen supply to be restored.

Similarly, uterine muscle needs another type of `food', calcium. Myometrial cells can store calcium, they use it up during a contraction and must then wait for internal stores to be replenished from outside the cell before being able to contract once again.

The Quenched Pregnant Uterus

The activity of the uterus is dampened down during pregnancy. Hormones are responsible for this change of state. In the early days of pregnancy hormones reach the tissue directly from the corpus luteum, in the early weeks a hormonal signal reaches the mother's hypothalamus telling her to secrete more progesterone, the hormonal flood she releases has side effects on other smooth muscle leading to nausea and constipation. Once the placenta has become established it can supply progesterone and beta-endorphin locally, i.e. where it is needed; the mother no longer needs to flood her whole body with progesterone and morning sickness usually passes. The main hormone responsible is progesterone but beta-endorphin may also be partly responsible. There is so much of it in pregnancy that at one time it was thought that only pregnant women secreted it. (Two hundred years ago giving opiates to a woman in labour was a recognised way of stopping contractions to give her a rest, although this k nowledge must have been forgotten today given the widespread use of pethidine, an artificial opiate, in labour.)

Contraction stopping hormones are very powerful; during pregnancy the uterus grows and stretches and stretches and grows and in the last trimester it does nothing but stretch. Given the usual propensity of smooth muscle to respond to stretch by contracting, the hormonal forces keeping the cervix shut are enormous.

In fact the stretch-contract reflex does operate during pregnancy but contractions are unable to spread to allow the extensive and extending coordinated contractions of labour. Stretch-contract reflex contractions will remain localised, enough to fold the baby back into the fetal position after he has stretched his legs but not enough to trigger labour. Throughout pregnancy the uterus is constantly flexing itself, getting into practice for labour. These contractions are strong enough in the last few weeks to have been given a name - Braxton-Hicks.

Contractions will start to spread when the oestrogen/progesterone ratio starts to favour oestrogen again and more electrical gap junctions are established allowing contracting muscle cells to influence adjacent muscle cells. But even if contractions do spread they will achieve nothing if the knot is still tied and cervical cells are contracting; the baby will be constantly folded back into the middle again.

The cervix again

Now we come back to the balloon. As long as the knot is firmly tied the air will stay in the balloon. If you press into a balloon the air does not come rushing out because it is a dosed system. The collagen knot at the cervix has the same effect. The uterus in pregnancy is a closed system, even nutrients are diffused through the placenta rather than there being a direct line from the mother.

The collagen `knot' is rather like the rolled over bit at the end of party balloons. You could not envisage stretching the end far enough apart to let a baby doll out - but cut off this end and this becomes easier to imagine. Dissolve the `knot' at the cervix (prostaglandins do this) and the uterus stops being a closed physical system and it becomes open at the cervical end. Now when contractions happen the baby starts to be eased downwards.

The Paradoxical Uterus

The uterus is wonderfully dynamic. During pregnancy it has been a safe solid container enfolding the fetus - his very own fortress and incubator. But during labour it becomes his ejector seat (all except the placental site which retains its protective role until he is ready for his lifeline to be cut off and to explore the world under his own steam). These two functions are diametrically opposed to each other. I find it utterly amazing that during labour the uterus performs both these functions at one and the same time - for of course the uterus must not cease its protective incubating role until the ejected baby starts to breathe for himself.

I hope that by now you will have a picture of a dynamic uterus, held in check by cervical `superglue' and hormonal `brakes' in the main body, but ready to labour once the hormonal milieu is favourable.

We can now return to the three major influences on the uterus in labour.

The mother's mind

I'm putting the mother's mind first because, after all, this is the ARM magazine and radical midwives know both by experience and by instinct that if you treat the mother well her labour will progress more smoothly; her body will usually act as nature intended. This is because the oxytocin secreted in labour is secreted on instructions from her pituitary which is under instructions from her hypothalamus, which is under instructions from her brain, which has integrated her perceptions of the world around her, interpreted them in terms of threat to her wellbeing and decided whether it is safe for her to continue to attempt to birth her child. What a very long chain of events - and it has glossed over one of the most important aspects - the social milieu. Woman is largely a social animal and her perception of the world includes the social environment - curtains and wallpaper in the labour ward are all very well but the people in that environment have far more influence over the mot her's mind. If she perceives them as threatening her uterus may refuse to function.

Labour is primarily a hormonal event, if too much contraction stopping hormone is secreted, the natural reflex action will be disrupted leading to uncoordinated, painful contractions which get nowhere. Note that labour hormones cannot be secreted by an act of will, the brain does not leave this aspect of reproduction, the life or death of the mother and child, to the mother's whim!

The mother's body

Even apart from the influence of the mind in labour, the mother is more than just a uterus on legs (at least we are `allowed' legs - i.e. mobility - in the 1990s). Her uterus is surrounded by the rest of her body, her bony spine, her diaphragm and so on. And why should the surrounding parts make any difference? The balloon metaphor together with the stretch-contract reflex shows that much of uterine activity in labour depends upon the basic forces of physics. We should never forget the influence of gravity. For a woman lying on her back the effect of gravity will be terrible - gravity will ensure that the baby will keep ramming his head into the wrong part of the uterus, and moreover, the tissue upon which his head impinges will not be able to be stretched so the stretch-contract reflex will not be available. Quite how the body manages to cope with this scenario I cannot image, it is surprising that labour is possible at all in these circumstances (and not surprising that resort to forceps or scalpel is so common).

And here we have a hint as to the role of the third party, the baby.

The baby

It was fundamental mistake to call the fetus the `passenger'. Just as, historically, doctors made the woman a passive object, so they have made her baby passive too. Calling the baby the fetus until he is actually born makes us liable to forget that he has an active part to play in his own birthing. He is anything but a passenger.

He moves.

When he moves he stretches his mother's uterus, initiating the stretch-contract reflex. While the waters are intact this stretch is gently diffused over quite a large area leading to a gentle contraction. Break the waters and the effect is concentrated in a much smaller area and the result is more painful. Any house agent will tell you that what is important is `location, location, and location', I'll just adapt this to `position, position and position' to make my point. When presenting normally, the baby's head guides him down his own birth canal. As his mother's uterus becomes ready to labour, as the hormonal brakes are taken off and the knot becomes untied, his head tells first this area then that area when to contract.

Now you must imagine a trampoline. Jump on it and it will throw you in a direction opposite to that in which you jumped, always steering you back into the safety of the centre. This is what the baby's head does in labour. It is an exquisite dance between uterus and baby.

Have you ever wondered why a newborn baby has all those reflexes which then disappear so soon after birth? Why bother to waste brain cells on a set of reflexes that will never be used in life? Because they are needed for birth itself.

Consider the stepping reflex. Could it be that the baby must `step' his way out of his uterine haven? The main mass of uterine muscle is the fundus. As the uterus contracts the fundal muscle pushes down and puts pressure on his feet which then react by pushing away which initiates another stretch-contract reflex and so on and so on.

Milani Comparetti (1981) lists four reflexes which peak at around the time of birth and fade away soon after. They are: the placing reaction, fetal locomotion (stepping or walking reflex), propulsion and the Moro reflex. He is convinced that they serve a purpose for labour; during pregnancy they allow the fetus to:

"move around and find the physiologic presentation by searching for the `invitation of softness' of the pelvic inlet into which he pushes his head with alternating rotations."

He suggests that the propulsion pattern is used for fetal collaboration in labour which then appears as the supporting reaction of the newborn. Milani Comparetti is aware of the implications for labour and he adds that cerebral palsy may be a cause rather than a consequence of dysfunctional labour, a point which should have been picked up by medical defence lawyers but appears to be unknown.


The powers

The labouring uterus is an amazingly powerful organ. The more I learn the more privileged I feel to have felt and experienced this power for myself but all too often in our hospitals the owner of those powers is not acknowledged. The uterus may be a powerful and dynamic organ, but without the say so from the brain it will not work efficiently. The uterus will try to do its best but if contraction-inhibiting hormones are interfering with cotraction-promoting hormones there is bound to be trouble - and pain, I think.

The passage

The textbooks' `passage' starts too far down in the pelvis. During labour the uterus opens up to become part of the passage. In order for the uterus to function at its best there needs to be far less constriction above the cervix. Absolute freedom of movement should be allowed - obstructions in other smooth muscle lead to excruciating pain, much of the pain in labour may be owed to constriction by the mother's own body. The uterus needs to be an open system from the fundus downwards.

The passenger

Of course the `passenger' is nothing of the kind. He brings his own innate neonatal reflexes into play to effect his own birthing. Instead of reluctantly submitting himself to pummelling by the powers of the uterus, the baby himself tells his mother where to contract the better to guide him into the world. When he is being starved of oxygen he protests vigorously, kicking against the uterus in an attempt to kick-start it again.

It seems to be a common observation that mothers delivering babies that have already died have a harder time in labour and the problems caused by malpresentation are common knowledge. Perhaps if the mechanisms of fetal reflexes were better known they could be used to help the fetus to turn himself into a better position before labour begins.

The mother's role

And the mother? Her body has to make greater efforts to stop her uterus contracting than to make it contract. Next time anyone mentions a `lazy' uterus to you remember this, it is harder work not to labour than to labour (if the uterus is indeed ready to labour, if it is not, you might as well give up). The mother has to do absolutely nothing except give the uterine and baby reflexes mental and physical space in which to act - that is, until her baby has passed through her cervix and his head is no longer impacting on her uterus. At this stage a maternal reflex takes over, a neuro-hormonal signal. Ferguson's reflex sends a message up for lots more oxytocin, which arrives some time later causing the expulsive contractions of the second stage, the baby moves down and presses against parts where the mother can feel him directly and she usually knows instinctively what to do.

The uterus has done its job. It merely has to wait for the baby's lifeline to stop being used so that the last inhibited area of uterus, the placental site, can be made active once more and reject the placenta. Then all is over bar the celebrations.

Time to blow up the balloons and start the party!


Cronk M (1997). Personal communication at proof stage of this article.

Milani Comparetti A (1981). `The neurophysiologic and clinical implications of studies on fetal motor behaviour', Seminars in Neonatology, 5, 2, 183-189.

Olah K S, Gee H and Brown J S (1993). `Cervical contractions: the response of the cervix to oxytocic stimulation in the latent phase of labour', British Journal of Obstetrics and Gynaecology, 100, 635-640.

Wray S (1993). `Uterine contraction and physiological mechanisms of modulation', American Journal of Physiology, 264, Cell Physiology, 33, C1-C18.


Childbirth Unmasked, my book where most of this picture of the uterus in labour was first unveiled was published four years ago. Before publication I approached an obstetrician, an expert on the cervix. He was quite unable (or unwilling) to give an opinion except to say that I was mistaken in thinking that the uterus had three different layers of muscle - he could see only one when he went into it to do a section.

Predominately male scientists are at a serious disadvantage; they have no hope of getting the fullest picture of uterine function; their perception of contractions is limited to what they can see on a screen. They hardly ever talk to mothers or midwives, from whom they would gain much valuable information.

Scientific disciplines have become more and more specialised and there is far too little cross-fertilisation between them. But as more information leaks out light begins to dawn, bits of the jigsaw puzzle of labour start to fit together. There is a long way to go before the picture will be complete but we could at least start to collect more pieces.

If anyone is interested, my book is still available via my address in the front of the mag; £8.95 including postage.

The Association of Radical Midwives.


By Margaret Jowitt

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