Amniotic Fluid Volume: too much, too little, or who knows?

Updated: May 2023

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 and I have provided references/links for research where I have stepped outside 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

Regulation of amniotic fluid in the second half of pregnancy

Amniotic fluid is in a constant state of circulation. In the second half of pregnancy, the primary 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. As the baby’s kidneys mature, urine becomes more concentrated. Therefore, lower volumes of urine are excreted into the amniotic sac.

Amniotic fluid volume reduces from 37 weeks gestation.

The amniotic sac and fluid play an important role during pregnancy and birth and you can read more about that in this 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.

You can find more information about amniotic fluid in pregnancy on my YouTube channel:

Accurate Measurement?

Here is the first problem… there is no accurate method for measuring AFV. There are two types of ultrasound tests aimed at assessing AFV:

  • Amniotic Fluid Index (AFI): four ‘pockets’ of fluid are measured by ultrasound and added up.
  • Maximum Vertical Pool (MVP): The ‘single deepest vertical pocket’ of fluid is identified and measured.

Both tests measure amniotic fluid in cms amniotic fluid with ‘normal’ values considered to be 5cm to 25cm. Studies comparing the two methods conclude that the ‘maximum pool’ measurement is the ‘better choice’ (Coombe-Patterson et al. 2017; Nebhan & Abdelmoula 2008Magann et al. 2011). The reasoning for this is because AFI increases the detection of oligohydramnios, resulting in increased rates of induction without improving outcomes for babies.

Measurement of AFV by AFI or ‘maximum pool’ is part of the Biophysical Profile assessment, which aims to identify 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 2017).

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 are no 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 the two types:

Uncomplicated oligohydramnios

This is mild oligohydramnios and occurs in an uncomplicated pregnancy. This is often associated with post-dates pregnancy and is caused by two factors:

  1. The normal physiological changes that occur to AFV as term approaches (see chart above) and/or the ‘normal’ level for the individual mother/baby are comparatively low to the general ‘norm’.
  2. Routine scans for post-dates pregnancy which then identify this normal AFV as ‘low’.

There is a lack of evidence to support induction for oligohydramnios in uncomplicated pregnancies ie. when there is nothing else ‘abnormal’ going on with mother or baby (Quiñones et al 2012). Driggers et al. 2004).

A 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 often leads to interventions such as induction.

Complicated oligohydramnios

Complicated oligohydramnios is generally a consequence of reduced urine output (baby) caused by reduced oxygenation. If the placenta is not providing adequate oxygenation, the baby attempts to compensate by redirecting blood flow away from the kidneys to the heart and brain. Therefore the oligohydramnios is a symptom of pregnancy complication such as pre-eclampsia or fetal growth restriction. It can also be caused by underlying congenital abnormalities of the baby. These babies often have very low AFV (easily identified by palpation), and are at significant risk. Further assessment and intervention should be offered.

Induction of labour is the usual recommendation for complicated oligohydramnios. However, induction needs to be considered carefully when the baby is already compromised by reduced oxygenation and limited glycogen stores. The induction procedure itself is associated with reducing placental circulation and causing hypoxia and fetal distress. Therefore, it is very likely that the baby will become distressed during labour before birth occurs. So, the mother must be prepared for, and informed of the likelihood of c-section. The other alternatives are awaiting spontaneous labour or a planned c-section. 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 consideration (and nerve) because time will not improve the situation, only worsen it. Even spontaneous labour is likely to result in fetal distress once contractions start because these babies are already struggling. Complicated oligohydraminios is a serious complication. I know a few women who have opted for a planned c-section rather than put their compromised baby through induced contractions.

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, infection, etc. But, usually no complication is present.

Complications associated with polyhydramnios

Most cases of polyhdramnios are mild and do not result in any complications. However, severe polyhydramnio can result in:

  • 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.

Management of polyhydramnios?

Tests may be suggested to see if a cause can be identified (although nothing can be done at this point). Mild polyhydramnios is best left alone because it is unlikely to result in any complications. When polydyramnios is severe, 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.

In Summary

  • 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 that need to be taken into consideration.

Further Resources

If you enjoyed this post, you can find more of my work in the following resources:

Join my Mailing List to stay up to date with new content and news