Episode 184 Too little amniotic fluid (oligohydramnios)

We explore what low amniotic fluid really means at different stages of pregnancy and when to act, when to watch, and how to personalise decisions. We share why routine induction for isolated low fluid may not improve outcomes and how to navigate monitoring, growth checks, and timing of birth.

• defining oligohydramnios and anhydramnios
• how gestation changes risk
• ruling out ruptured membranes with history and tests
• expert scans, infections, and maternal factors
• why fluid matters for lung development
• isolated low fluid late in pregnancy
• induction evidence and unplanned caesarean risk
• growth surveillance and timing around due date
• continuous heartbeat monitoring considerations
• practical reassurance and shared decision-making

Want to know more:

https://obgyn.onlinelibrary.wiley.com/doi/10.1111/tog.70021

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Thank you all for listening,  My name is Florence Wilcock I am an NHS doctor working as an obstetrician, specialising in the care of both mother and baby during pregnancy and birth. If you have enjoyed my podcast please do continue to subscribe, rate, review and recommend my podcast on your podcast provider.
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Transcript

Welcome And Topic Setup

Florence 0:00

Hello, my name's Florence. Welcome to the Obspod. I'm an NHS obstetrician, hoping to share some thoughts and experiences about my working life. Perhaps you enjoy Call the Midwife. Maybe birth fascinates you, or you're simply curious about what exactly an obstetrician is. You might be pregnant and preparing for birth. Perhaps you work in maternity and want to know what makes your obstetric colleagues tick, or you want some fresh ideas and inspiration. Whichever of these is the case, and for that matter anyone else that's interested, the obspod is for you. Episode 184, Too Little Amniotic Fluid. Last episode we talked a lot about excessive amniotic fluid or polyhydramnis, and that was off the back of an article review article that I had read, which I included in the show notes. Today I'm going to cover the rest of what that article said, and that was relating to too little amniotic fluid or oligohydramnis. So just as with too much amniotic fluid, first of all we need to have some sort of definition about what we mean by too little amniotic fluid. Remember we talked about amniotic fluid index measurements or single deepest pull measurements. Whereas anhydramnis is defined as a complete lack of fluid. And then a little bit like with excessive amniotic fluid, the meaning of too little amniotic fluid depends on what gestation, what point in pregnancy is this picked up. So if this is picked up relatively early in pregnancy, say at the 20-week anomaly scan, then this is highly suggestive that there is a major problem with this pregnancy. Remember, I said the fluid is constantly being swallowed and recirculated, so a lack of amniotic fluid could mean something that means the baby's not producing any urine, such as lack of the formation of normal kidneys and bladder, or suggestive that the placenta is not functioning properly at that early stage of pregnancy. But the first thing we need to consider at any stage of pregnancy when there's a lack of or reduced amount of amniotic fluid is the possibility that the woman could be losing fluid, so her waters could have broken. And so the first thing we'll do is ask women have they been leaking any fluid, and usually do an examination with a speculum, a bit like a smear test, to look for any amniotic fluid in the vagina and do a little test called an amnisure test, which will test for the presence of amniotic fluid in the vagina. If you've watched people breaking their waters on television, there's a massive gush and splash and it's all over the floor. Well, actually, breaking your waters isn't always like that, it can just be a little trickle and not necessarily something that every woman will obviously notice. And when we talk to women, usually when they're losing fluid, they definitely feel more than damp, they feel wet, and you can often tell from taking a good history, having a good conversation with a woman, but you can't always. Some women we never figure out when it was those waters went. So we need to take a detailed history of what she's been experiencing over the past few days and weeks and perform an examination. Once we've ruled that out, again, depending a bit on what stage of pregnancy this lack of fluid is picked up at, much as with extra fluid, you'll probably be referred for a fetal medicine scan, an expert scan, to do a top-to-toe check of the baby. Again, probably checking for infections, checking for anything abnormal about how the baby's formed, and then there may be some maternal conditions such as raised blood pressure, which could be contributing to the lack of fluid around the baby. If early on in pregnancy there's a lack of fluid around the baby, this is very serious and can have an associated poor outcome. And this is partly because the baby needs that fluid around it, that cushion around it for some of its normal development. For example, the baby's lungs need fluid to develop. So when your waters break extremely early or the amnipositic fluid hasn't formed properly round your baby, you will have some very serious conversations with doctors about what the likely outcome for your pregnancy might be. But just as with severe polyhydramnius, severe excessive fluid being rare, and hydramnius or the lack of fluid or early onset lack of fluid is also extremely rare. What we're going to see much more commonly is isolated lack of fluid or reduced fluid, which complicates between half a percent and five percent of pregnancies. Again, we want to rule out growth problems with the baby or an infective problem with the baby, but once we've done that, so this is called a diagnosis of exclusion, where you've excluded all the things you you do know about, then you can assume it's isolated or idiopathic oligohydramnis. So then we have to think, and this is particularly the case if it happens later in pregnancy. So then we have to think what is the relevance of this? And traditionally, we've always said that a lack of fluid, if if the waters haven't broken, a lack of fluid even with a normally grown baby could indicate that the placenta is starting to function less well, and that this could represent the beginning of what we call placental insufficiency. The placenta's not working as well, and therefore we need to consider whether or not we intervene to deliver the baby a little bit earlier than we might otherwise have done. And when we're thinking about early delivery, we may be discussing induction of labour or we may be discussing caesarean birth. And one of the things we'll be concerned about is the higher chance of the baby being stressed during labour because there's less water, there's less cushioning around the baby, and therefore more chance of changes in the baby's heart rate due to umbilical cord compression, that is the umbilical cord being pressed on as the baby journeys down the birth canal or being pressed on during contractions. So there definitely is a school of thought that would recommend continuous heartbeat monitoring of any baby in a pregnancy where reduced amniotic fluid has been picked up. So often in my career I've had a conversation with a woman who's considering what to do, and there's been reduced water around the baby, and I've had to say I think this could be a sign that the placenta's not working as well, and that actually your baby would be better being born sooner rather than later. How about induction of labour? And so it interested me to read in this recent review article that actually there isn't clear evidence that if you induce labour in cases of isolated oligohydramnis, isolated reduced water round baby, that it improves the outcome for babies at all in terms of what we call perinatal morbidity and mortality. And this could be especially the case where you're doing a scan and the baby's normally grown and then there's just reduced amniotic fluid, because if the baby's growth is normal, then there's little to suggest that this is something to do with the placenta. So they concluded that if you have less water around the baby and you induce labour, the baby's normally grown and you induce labour just because of the reduced amount of fluid around the baby, that there's an increased chance of an unplanned emergency caesarean birth because of perhaps concerns about the baby's heartbeat monitoring, or because we as clinicians have a lower threshold for intervening because we've already got in the back of our mind is there something wrong with this placenta, and we're expecting some cord compression during labour. Interestingly, they commented that there's no consensus in the major clinical practice guidelines, both the Royal College of Obstetricians and Gynecologists in the UK or America or Europe. So they're suggesting that a large multi-centred trial might be of benefit. So the review article suggested that birth should be individualised, the decision to intervene should be individualised depending on the woman's other risk factors. And until more robust evidence could be produced, that a pragmatic approach would be to induce labour around term, so 40 weeks. What did seem to be clear though that if you pick up reduced amniotic fluid around a baby in pregnancy, there is a higher chance of a growth problem for the baby, and therefore it is sensible to offer some additional scans during the remainder of the pregnancy, an inch increased vigilance, as it were. Also, that even if the baby appears to be normally grown, you may find because of our margin of error in measuring babies on scans, you might actually find that the baby is slightly growth restricted or slightly smaller than it was destined to be after birth. So the authors of the article suggest that you individualise care but you do offer continuous heartbeat monitoring of the baby, and that it would be sensible for such babies to be born around about their due date rather than overdue, and that access to neonatal services should be available. So, did reading this article make me think about how I might change my practice? Well, it definitely suggested that oligohydramnia, so lack of water or reduced water around a baby at term at the end of pregnancy might be more common than I had realised, and more common in the absence of any other signs of anything untoward in the pregnancy. So it's definitely made me reflect and think about that. It's also made me think about our assumption that we make about the likely chance of the baby being a bit smaller or growth restricted and the placenta not functioning as well. And it doesn't seem that there is good evidence that that is the case. So it's made me slightly re-evaluate perhaps how urgently or not I need to suggest that we intervene in the pregnancy and get this baby born. So what is my zesty bit? I think my zesty bit is if you have an incidental finding on a scan late on in pregnancy of reduced amniotic fluid around your baby, it's not a big deal. You probably will be referred for some extra scans and blood tests and to have a conversation with an obstetric doctor to think about your options. But the idea that this means definitely your baby is in trouble is simply not the case. You do need to think back and think: could you have lost some water, could you be losing fluid? And the doctors and midwives will want to examine you and check you for that. But I think that this finding on scan is nothing to be alarmed about, but is some important information that you need to take into consideration when thinking about what health professionals are recommending. And if you're a midwife or doctor seeing a woman with reduced fluid on scan, I think again don't knee-jerk into taking intervention because there's not a lot of evidence that this is necessarily beneficial to either mother or baby. If you've got significantly reduced amniotic fluid at your 20-week scan, this is a very different situation, and you will be seeing a fetal medicine specialist, a specialist scan consultant who can talk you through all the possible options as to why this may be happening to your pregnancy. And I'm going to talk a little bit about breaking waters very early and what that means for pregnancy in a future episode. I very much hope you found this episode of the Obspod interesting. If you have, it'd be fantastic if you could subscribe, rate, and review on whatever platform you find your podcasts, as well as recommending the ObsPod to anyone you think might find it interesting. There's also tons of episodes to explore in my back catalogue from clinical topics, my career and journey as an obstetrician, and life in the NHS more generally. 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