It is fairly common for women to experience a phenomenon known as ‘pre-labour rupture of membranes’ or PROM prior to labour starting, but it can be surprisingly tricky to work out what is happening when a woman starts leaking fluid via the vagina. I’m talking here about an otherwise normal pregnancy which has got to 37 weeks gestation or more. Why is anyone concerned about this? Surely it’s a good thing as prelabour rupture of membranes is known as a sign that labour will often start soon. Indeed, the majority of women who experience prelabour rupture of membranes will go into labour within 24 hours. (See this blog for further discussion of the research evidence: https://www.evidentlycochrane.net/prelabour-rupture-membranes-induce-wait/).
However, one of the roles of the intact sac of fluid (a balloon-like structure filled with amniotic fluid known as the ‘waters’) is to protect the baby from bacteria (and hence infection) that may be introduced via the vagina. So it’s important to be able to correctly identify if the membranes are intact (in which case the likelihood of infection is not increased), versus a rupture of membranes where the sac of fluid surrounding the baby does have a rupture (break), (because if the sac is truly ‘broken’ there may be a raised possibility of bacteria gaining access to the fluid that surrounds the baby).
If labour doesn’t start within 24 hours it has become standard practice in the UK for a woman to be offered induction of labour* in order to reduce the risk of infection. This is not an uncontroversial approach, because even with a known rupture of membranes, infection rates actually rise slowly over the first 96 hours. You may like to check out the Ontario midwives’ research evidence-based guideline as an alternative to the NICE guidance on this issue: Ontario Midwives guideline. However, my main focus in this article is not induction of labour itself, but whether all apparent ruptures of membranes are in fact true ruptures even where amniotic fluid is released. I know this sounds mysterious, even impossible – please bear with me while I explore three of the most common scenarios.
Scenario One: the obvious rupture of membranes
Sometimes it’s easy to understand what is happening and we have an obvious rupture of membranes. The ‘waters’ release with a gush and sometimes women feel or even hear a ‘pop’.This makes it pretty obvious what has happened. The waters are flowing out copiously. The woman feels as if she is constantly losing fluid, she’s unable to stop it, and she’s regularly soaking through pads or even towels. The fluid may be straw coloured, light pink or possibly stained green or brown if baby has passed meconium (the first poo).
Scenario Two: the weeping cervix
Then we have the scenario where there is a trickle of watery secretion, which may be enough to cause a damp patch … but there isn’t a continuous flow, and any trickles are usually not sufficient to soak a pad.There are tests that can be done nowadays that help to identify whether this is amniotic fluid or not and if this proves not be amniotic fluid, we can assume this is actually a watery secretion that is very common as the cervix ripens. I first heard this described by independent midwife Jane Evans. She explained that the cervix changes consistency as it ripens; indeed the cells themselves are changing their structure as labour approaches and these changes cause the cervix to ‘weep’. It can be a bit disconcerting to leak fluid in this way, but if tested with a home test such as Amniosense pads (available in larger Boots stores) or tested in a clinic, there will be no doubt that there is no amniotic fluid and the membranes remain intact.
Scenario Three: Hindwater rupture?
There is a third scenario. There seems to be a fairly obvious rupture of membranes. A pad is soaked, or two pads, with no surges (contractions) – labour has not started. Then the fluid trickle slows or even stops. Sometimes there is a minor trickle after the initial flow. If tested, the fluid tests as amniotic fluid. The woman is recommended by her midwife or obstetrician to have an augmentation (induction) of labour after 24 hours if the labour hasn’t started by itself. As part of the induction process, a vaginal examination is conducted. If the cervix is dilated (open) enough, it is possible to feel the sac (bag) of fluid bulging through the opening. It feels like a balloon full of water. It is usually possible to feel the harder shape of the baby’s head lying behind the balloon.
It seems odd if there has been a rupture of the membranes that the bag of membranes should be bulging, but the midwife or obstetrician has an explanation.The woman is told that she must have had a ‘hindwater rupture’. The explanation is that the membranes have sustained a rupture in a part of the balloon-like sac behind the head. The head acts as a plug so only small amounts of fluid escape at a time. The membranes that bulge in front of the baby’s head are described as the ‘forewaters’. As part of the induction process, the midwife or obstetrician uses an amnihook (like a crochet hook) to make a hole in the membranes (this is known as amniotomy). The waters often flow out in copious amounts at this point and the balloon-like sac seems to deflate. Usually this brings the baby’s head down onto the cervix and often stimulates surges (contractions) within a few hours.
This description of what has happened is told to women as if the hindwater/forewater description is a fact. However it is actually a theory rather than a fact as far as I’m aware. However, I’ve come to believe that there may be another explanation.
Is the so-called hindwater rupture in fact a rupture of the chorion? I need to explain a bit more about the anatomy of the amniotic sac in order to describe what I mean.
Scenario four: chorion rupture leaving the amnion intact
The baby lies within a balloon-like structure known as the amniotic sac which is filled with fluid called amniotic fluid, commonly known as the waters. The walls of the sac consist of two membranes with the outer membrane of the sac being called the ‘chorion’ while the inner membrane is called the amnion. It is the amnion layer that produces the amniotic fluid in which the baby floats and swims. (See http://www.pregmed.org/amniotic-sac.htm for more details.)
I recall a few years ago, I cared for a woman planning a home birth who experienced the supposed ‘hindwater’ scenario. She had a fairly substantial flow of clear fluid which soaked a pad or two but which then diminished. We did not test the fluid that was escaping – there seemed to be a good history of a spontaneous rupture of membranes or SRM. However, the flow virtually ceased over the next few days yet there was no sign of labour. The woman ended up requiring a vaginal examination (not recommended unless absolutely necessary if it is suspected the membranes have ruptured) and I felt the taut bulging membranes which were positively bulging with fluid. I could feel quite clearly that there seemed to be a hole in the chorion – I could feel an edge to the ‘hole’ which could be moved (the chorion slipping over the amion). A lightbulb went off in my head – the chorion had ruptured, but the amnion was intact!
When the amnion did rupture (one of the very few times in my career as an independent midwife I did an ARM) there was a very substantial flood of fluid and I seem to remember that baby was born about an hour later.
I had never heard of such a thing as a chorion-only rupture, but I undertook a google search and I came across a single abstract by J S Cohain (see https://www.ncbi.nlm.nih.gov/pubmed/25279443) which describes this phenomenon as ‘false’ rupture of membranes. I haven’t accessed the full paper but the abstract doesn’t seem to suggest what I’m suggesting – that possibly some/many cases of supposed hindwater rupture may actually be a rupture of the chorion, while the amnion itself remains intact.*
If the amnion is intact, how can amniotic fluid be leaking? To answer this, we need to understand more about how amniotic fluid is made and how it is held within the amniotic sac in normal circumstances – in other words, why does the amniotic fluid secrete from the membranes into the space around the fetus, but does not usually escape the other way? How do the amnion and chorion together usually perform the function of the balloon-like structure where fluid is held within the sac but does not escape on the ‘maternal’ side of the sac?
I do not claim in any way to be a physiologist or anatomist and that is why I am writing this informally as a blog, rather than sending it to an academic journal. I hope these thoughts may inspire further discussion and investigation. Having said that, I do know that the membranes have both exude fluid (created initially from maternal serum), and absorb and remove excess fluid in the most amazing way. The fetus plays a big part in regulation of fluid balance, by swallowing and urinating. But the fine balance is maintained by the membranes themselves, secreting and reabsorbing this incredible substance of amniotic fluid. My supposition is that towards the end of pregnancy, when the membranes naturally become more fragile in preparation for the birth when usually they rupture (if not interfered with) at some point just prior to or during labour, during this time it is possible that there may be a breach in the chorion while the amnion remains intact. The chorion would normally help to return excess fluid to the maternal serum, acting as a kind of seal and helping to prevent leakage of fluid on the maternal side of the barrier. However, the seal is not complete if the chorion is not complete, and therefore some small amount of fluid may be able to leak across the still intact barrier of the amnion via osmosis and be released via the vagina. I think this explains why, after an initial gush (which is still nowhere near as big as when both membranes rupture at the same time), the trickle is small but continuous, and ultrasound scan shows normal liquor volume amounts for that stage of pregnancy.
I emphasise that this is in later pregnancy; we do know that in earlier stages of pregnancy a tiny rupture can seal itself again. I think sealing of a small breach in either membrane or both in late pregnancy is less likely, as the cellular structure of the membranes is altering – weakening – in preparation for the birth. However, this is one of many things of which I cannot be sure.
Hindwater rupture or chorion rupture: does it really matter?
I think it does matter, because if there has been a true rupture of both chorion and amnion this means there is a breach in the sac surrounding the baby which could potentially allow bacteria to enter the fluid directly around the baby. This fluid is intended to be sterile until the baby is born in order to protect the baby from infection because the baby does not have a developed immune system of its own until many weeks after it is born (though baby does have some secondary immunity from the mother).
The more I think about it, the more I question the logic behind Scenario Three – the standard hindwater rupture. It is supposed that a small hole occurs in the membrane behind the baby’s head and the baby’s head acts like a cork. But actually, the baby’s head isn’t a cork – it doesn’t fit totally snugly. Why, after the initial flow of fluid, is there so little? If there was any kind of breach in the sac wouldn’t you expect a steady stream – water doesn’t need a big space through which to leak? And when the supposed ‘forewaters’ do eventually rupture, whether artificially or naturally, there are often copious quantities of fluid – far more than would be expected from these forewaters which are supposed to be just an inch or so of fluid filled structure. Why is there so much fluid at this stage if there has previously been any kind of true rupture?
Doesn’t it seem more likely that the chorion has ruptured, which allows a relatively small amount of clear fluid to release at the time of rupture, but which then almost stops leaking?
Is it possible that there is no such thing as a hindwater rupture?
Wouldn’t this be extraordinary? Could hindwater rupture fall into the category of medical myths which seem self-evident at the time, but which later generations view as laughably naive?
Let’s hope so, because if my theory is correct and most cases of hindwater rupture are in fact chorion ruptures, then the amnion is still intact for these mothers and babies and (though this has not been tested to my knowledge) it may be there there is less likelihood of bacteria entering the amniotic fluid.
I’d love to be able to test whether this theory holds water. (Forgive the dreadful pun!) I’d be very grateful if you could comment to let me know your thoughts about the theory and about how we could assess whether there’s any truth in it. One way that’s occurred to me may be a research study where we ask midwives to make careful examination of all amniotic sacs after the birth and audit the findings, with a view to seeing if we can identify where the breaks are (very difficult but it may be possible at least in some cases). We could then build on this knowledge and see if we can prove or disprove the hindwater rupture theory. We can try to work out a way to differentiate between a rupture of chorion and amnion or chorion alone. And maybe we can prevent some unnecessary inductions of labour. In these days where induction of labour is ever-increasing, it may be a goal worth pursuing.
*I use the term induction for clarity, though strictly speaking if there has been a rupture of membranes the term used among midwives and obstetricians is ‘augmentation’.
*Edited 21/04/2021 after a Twitter discussion with Joy Horner