Updated: June 2018
This post is in response to readers asking me to cover the topic of induction for low amniotic fluid volume (AFV). Most of the content is available in textbooks, in particular Coad and Dunstall 2011 and Beall and Ross (2011), and I have provided references/links for research where I have stepped outside the textbook sources. I use the word ‘may’ quite a bit in this post because little is known about AFV, therefore a lot of the available information is theoretical. In fact, this post probably raises more questions than answers! Also note that I am focussing on AFV at term.
Amniotic fluid volume regulation
Amniotic fluid is in a constant state of circulation. In the second half of pregnancy the main sources of fluid production are from the baby:
- urine (700mls per day)
- lung secretions (350ml/day)
And the main sources of fluid clearance are:
- the baby swallowing fluid and passing it back into mother’s blood stream
- direct flow across the amnion (placental membrane) into placental blood vessels
The balancing act required to maintain a healthy AFV may be influenced by hormones (prolactin and prostaglandins), osmotic and hydrostatic forces, and the baby. Maternal hydration is also associated with AFV (Patrielli et al. 2012). From day to day there is little change in AFV, however volume decreases towards term. This is normal. Perhaps this reduction reflects reduced lung secretions as the baby nears term?
The amniotic sac and fluid play an important role during pregnancy and birth – you can read more about that in an earlier post.
Abnormalities in AFV occur when there is an imbalance between fluid production and clearance. Too much fluid is called ‘polyhydramnios’ and too little fluid is called ‘oligohydramnios’. However, measurement and thresholds of normal/abnormal are not clear.
Here is the first problem… there is no accurate method for measuring AFV. The two ultrasound tests aimed at assessing AFV are:
- Amniotic Fluid Index: four ‘pockets’ of fluid are measured by ultrasound and added up resulting in an Amniotic Fluid Index (AFI) eg. AFI = 10cm.
- Maximum Pool: The ‘single deepest vertical pocket’ of fluid is identified by ultrasound and measured in centimetres.
Neither of these methods are supported by research (that I can find). However, studies comparing the two conclude that the ‘maximum pool’ measurement is the ‘better choice’ (Nebhan & Abdelmoula 2008; Magann et al. 2011). The reasoning for this is interesting… AFI increases the detection of oligohydramnios resulting in increased rates of induction without improving outcomes for babies. So the best method is the one that does not detect the ‘problem’ you are looking for?
Measurement of AFV by AFI or ‘maximum pool’ is part of the Biophysical Profile assessment which aims to identifying babies with inadequate oxygenation via the placenta. However, it is unclear whether there is any benefit to this test. Indeed, an umbilical artery doppler test may provide a better assessment of placental function, and therefore how well oxygenated the baby is (Alfirevic, Stampalija & Gyte 2010) – which is what everyone is worried about.
There is of course the old fashioned method of assessment, also not well researched. Abdominal palpation is usually carried out during antenatal visits. In addition to working out what position the baby is in, a midwife assesses the amniotic fluid volume. When you have palpated lots of pregnant bellies, ‘real’ polyhydramnios and oligohydramnios are usually pretty obvious. Mothers are also experts regarding their own body/baby and notice differences themselves – particularly if they have been pregnant before and can compare pregnancies. What you may find:
- Oligohydramnios: baby is very easy to feel – in some cases you can see limbs; the uterus is smaller than expected; the mother may notice reduced movements.
- Polyhydramnios: baby is difficult to palpate and floats away as you apply pressure; the uterus is bigger than expected; the baby’s heart rate may sound muffled; the mother may notice breathlessness, vulval varicosities, oedema and gastric problems.
When you are working as a midwife in a continuity of care situation you get familiar with the individual woman’s bump over time, and it is easier to notice changes. Measuring (with a tape measure) is often used to assess uterine growth – particularly when care is spread between a number of practitioners. Whilst measuring can assist with identifying polyhydramnios, it is unreliable in identifying oligohydramnios (Freire et al. 2013).
Here is the second problem… there is currently no agreement about what constitutes ‘high’ or ‘low’ levels of AFV. Megann et al. (2011) conclude that: ‘high and low levels [of amniotic fluid] have yet to be established in the literature and are difficult to directly link to adverse pregnancy outcomes.’ So we are busy finding measurements that we don’t really understand the implications of?
Most of the time there is no known cause for the ‘high’ or ‘low’ volume of fluid, and there is are complications caused by it. However, there are some factors worth considering if you are labelled with oligohydramnios or polyhydramnios.
Oligoydramnios – too little
The definition of oligohydramnios is usually less than 500mls of fluid; <2cm maximum pool; or AFI <5. Around 3-5% of pregnant women are diagnosed as having too little fluid. Because of the complexities of measurement and the diagnosis of oligohydramnios, I have differentiated between what I believe are two types:
Most cases of ‘oligohydramnios’ are an outcome of 2 factors:
- The normal physiological changes that occur to AFV as term approaches (see chart above) and/or the ‘normal’ level for the individual mother/baby is comparatively low to the general ‘norm’.
- Women having routine scans for ‘post-dates’ which then identifies this normal ‘low’ AFV.
There is a lack of evidence supporting induction for oligohydramnios in ‘low risk’ pregnancies ie. when there is nothing else ‘abnormal’ going on with mother or baby (Quiñones et al 2012). Driggers et al. (2004) concluded that: “evidence is accumulating that in the presence of an appropriate-for-gestational age fetus, with reassuring fetal well-being and the absence of maternal disease, oligohydramnios is not associated with an increased incidence of adverse perinatal outcome.’’
A recent review of the literature (Rossi & Prefumo 2013) found that in term or post-term pregnancies oligohydramnios (with an otherwise healthy pregnancy/baby) was not associated with poor outcomes. However, it was associated with increased risk of obstetric interventions… probably because the diagnosis leads to intervention.
Pathological oligohydramnios is generally a consequence of reduced urine output (baby) which can indicate a redirection of blood flow away from the kidneys to the vital organs in response to reduced oxygenation. This usually occurs alongside pregnancy complications such as pre-eclampsia. In this case the low fluid volume indicates inadequate placental circulation to the baby. These babies often have significantly low AFV (easily identified by palpation), and are often growth restricted i.e. small and with limited glycogen supplies. These babies are at significant risk and further assessment and intervention should be offered. As a midwife there are few things more concerning than being able to see baby’s form though their mother’s abdomen.
Induction of labour is the usual management for oligohydramnios (regardless of type) because there is concern that the baby has inadequate placental circulation (which is correct in relation to pathological oligohydramnios). However, women need to consider that the induction procedure is associated with reducing placental circulation and causing hypoxia and fetal distress. In addition, if the baby is post-dates he may have already passed meconium, and/or will if he becomes hypoxic due to the induction process. Oligohydramnios = less fluid to dilute the meconium = increased risk of meconium aspiration. Therefore, it is very likely that the baby will become distressed during labour before birth occurs. This is why electronic fetal monitoring is important – in clinical practice I saw the inevitable fetal distress and rush to theatre resulting from induction for oligohydramnios over and over again. So, the mother must be prepared for, and informed of the likelihood of c-section. The other alternatives are planned c-section or awaiting spontaneous labour. Whilst spontaneous labour is more gentle on the baby than an induction, waiting for labour with a baby who is not being well supported by their placenta requires serious very consideration (and nerve) – time will not improve the situation, only worsen it. Even spontaneous labour is likely to result in fetal distress once contractions start – these babies are already struggling. Pathological oligohydraminios is a serious complication.
Polyhydramnios – too much
The definition of polyhydramnios is usually around 2000mls of fluid; >8cm maximum pool; or AFI >25cm. Around 1-3% of pregnant women are diagnosed with having too much amniotic fluid. In 60% of cases there is no known cause, but factors that increase fluid volume include:
- The baby producing too much urine
- Decreased fetal swallowing (baby)
- Increased water transfer across the placenta by the mother
These factors may be influenced by the general well being of mother and baby ie. may occur if there are complications present such as diabetes, rhesus isoimmunisation, congential abnormalities, etc. But, usually no complication is present.
Complications associated with polyhydramnios
- Preterm birth – as the uterus become over stretched with fluid.
- ‘Unstable’ position of the baby – the baby can float about into helpful and not so helpful positions.
- Cord presentation or prolapse – because the baby is floating about the cord can get between his head and the cervix.
- Placental abruption – may occur with a sudden change in fluid volume and therefore size of uterus/placental site.
Tests may be suggested to see if a cause can be identified (although nothing can be done at this point). Induction of labour with a ‘controlled’ artificial rupture of membranes may be suggested to manage the risk of an unstable lie and/or cord prolapse. This involves breaking the waters whilst holding the baby in place… and with quick access to theatre as the procedure can result in a cord prolapse. Alternatively, the woman may choose to wait until labour begins, and assess her baby’s position once contractions have started. Either way – the risk is the woman’s therefore she must be the person to decide which risks are best for her – induction or waiting.
- The exact mechanisms involved in regulating AFV are still unknown.
- AFV reduces significantly after 37 weeks – this is normal.
- There are no accurate methods of measuring amniotic fluid.
- There is no agreement about what measurements indicate ‘high’ or ‘low’ AFV.
- The intervention used to manage polyhydramnios or oligohydramnios ie. induction also carries risks which need to be taken into consideration.
So, as you can see this topic creates more questions than answers which is why I previously avoided it!
Journal article (Zhang et al 2017) re. the complex function of amniotic fluid in mother-baby communication