Amniotic Fluid Volume

intervention physiology

Induction of labour is often recommended when amniotic fluid volume (AFV) is estimated to be too high or too low. However, little is known about amniotic fluid regulation or the parameters of normal AFV. This post explores what is known and unknown and whether induction based on AFV improves outcomes.

Amniotic Fluid Volume Regulation

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)

The primary sources of fluid clearance are:

the baby swallowing fluid and passing it back into the mother's bloodstream
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 [1]. From day to day, there is little change in AFV. However, volume decreases towards pregnancy. This is normal. As the baby's kidneys mature, urine becomes more concentrated. Therefore, lower volumes of urine are excreted into the amniotic sac.

The amniotic sac and fluid play an essential role during pregnancy and birth; 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, measurements 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 of 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' because AFI measurement increases the detection of oligohydramnios, resulting in increased induction rates without improving outcomes for babies [2,3,4].

Measurement of AFV by AFI or MVP is part of the Biophysical Profile assessment, which aims to identify babies with inadequate placental oxygenation [5]. However, it is unclear whether there is any benefit to this test. An umbilical artery Doppler test provides a better assessment of placental function and therefore, how well-oxygenated the baby is [6].

There is, of course, the old-fashioned method of assessment, which is also not well-researched. Abdominal palpation is usually carried out during antenatal visits. In addition to working out tn, a midwife assesses the amniotic fluid volume. When you have palpated lots of pregnant bellies, significant polyhydramnios and oligohydramnios are usually pretty obvious. Mothers are also experts regarding their body/baby and notice differences themselves, particularly if they have been pregnant before and can compare pregnancies. What you may find:

Oligohydramnios: The baby is very easy to feel, and in some cases, you can see limbs; the uterus is smaller than expected; the mother may notice reduced movements.
Polyhydramnios: The 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 several practitioners. Whilst measuring can assist with identifying polyhydramnios, it is unreliable in identifying oligohydramnios [7].

The second problem is that there is no agreement about what constitutes 'high' or 'low' levels of AFV. Megann et al. [7] 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, some factors are worth considering if you are labelled with oligohydramnios or polyhydramnios.

Oligohydramnios – 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:

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'.
Routine scans for post-dates pregnancy identifying this normal AFV as 'low'.

There is a lack of evidence to support induction for oligohydramnios in uncomplicated pregnancies, i.e. when there is nothing else 'abnormal' going on with the mother or baby [8,9].

A review of the literature found that in term or post-term pregnancies, oligohydramnios (with an otherwise healthy pregnancy/baby) was not associated with poor outcomes [10]. 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 their kidneys to their heart and brain. Therefore, oligohydramnios is a symptom of pregnancy a complication such as pre-eclampsia or fetal growth restriction. Underlying congenital abnormalities of the baby can also cause oligohydramnios. Complicated oligohydramnios results in very low AFV (easily identified by palpation), and 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 a 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 oligohydramnios 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, i.e. it may occur if there are complications present, such as diabetes, rhesus isoimmunisation, congenital abnormalities, infection, etc. But, usually, no complication is present.

Complications associated with polyhydramnios

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

Preterm birth – as the uterus becomes overstretched 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 their head and the cervix.
Placental abruption – may occur with a sudden change in fluid volume and, therefore, the size of the 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 polyhydramnios 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.

Summary

The exact mechanisms involved in regulating AFV are still unknown.
AFV reduces significantly after 37 weeks, which 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, i.e. induction also carries risks that need to be considered.