Most pregnant women don't even think about their baby's amniotic fluid levels, until their healthcare provider tells them it could be a problem.
Some women are told they have too little or too much amniotic fluid.
But what does this mean for a mother and her unborn baby?
This article has been designed to explain everything you need to know in simple terms, should the topic of amniotic fluid levels be put on the table.
Amniotic Fluid Levels
During pregnancy, as your baby grows and develops inside the uterus, he is protected by the amniotic sac and fluid inside. This fluid doesn’t get much thought until you are full term, and worrying about your membranes breaking in the middle of aisle three at the supermarket!
Yet amniotic fluid plays a very important role in your baby’s growth and development. It also acts as a buffer to protect your baby from external forces and injury. How much fluid should be present can become a concern for some women during pregnancy.
What Is The Amniotic Fluid?
Within days of an egg being fertilized, the amniotic sac begins to form and fill with fluid. This fluid is clear and pale straw-coloured, and is initially created from the mother’s plasma (pale yellow liquid component of blood).
Until the 12th week of pregnancy, amniotic fluid is mostly water with electrolytes. In the second trimester proteins, carbohydrates, lipids and urea are present, which aid in the growth of the baby.
From around 16 weeks of pregnancy, the baby’s kidneys begin to function, and fetal urine becomes the main source of amniotic fluid. The other source of amniotic fluid is fluid excreted from the baby’s lungs.
What Is the Purpose Of Amniotic Fluid?
Initially, the amniotic fluid is absorbed by the baby’s skin and tissue. At around 20 weeks of pregnancy, the skin begins to change, and the fluid is ingested rather than absorbed.
Amniotic fluid acts as a buffer to protect the developing baby, cushioning against any bumps or injury. It also allows for easy movement, which promotes muscular and skeletal development.
Amniotic fluid swallowed by the baby helps to form the gastrointestinal tract. Swallowing is an important developmental skill babies practice in utero for many months, in preparation for breastfeeding after the birth. The fluid forms urine and maintains a constant temperature for the baby.
How Much Amniotic Fluid?
As the baby grows it produces more amniotic fluid. The amount increases until the baby is about 32 weeks gestation. The amniotic fluid levels then remains constant until the baby is full term (37 to 42 weeks) when the levels start to decline. Small changes in the level occur as the baby swallows the fluid.
In some pregnancies, there may be too much (polyhydramnios) or too little (oligohydramnios) amniotic fluid. These changes in amniotic fluid levels are often a cause of concern for care providers, as it indicates a problem in the balance between fluid production and clearance.
How Is Amniotic Fluid Measured?
There are two ultrasound measurements used to measure the amniotic fluid levels surrounding the baby:
- Amniotic Fluid Index (AFI): this method uses the sum of four of the deepest vertical pocket of fluid. The normal range for the AFI is 5-25 cms.
- Single Deepest Pocket (SDP): this method measures the depth of the largest visible pocket of fluid surrounding the fetus. The normal range for maximum pool is 2-8 cms.
A review comparing AFI and SDP found the use of AFI results in over diagnosis of too little fluid (oligohydramnios). This leads to unnecessary interventions such as induction, which contributes to increased risk of injury or death, without any improvement in perinatal outcomes.
Both AFI and SPD are qualitative measures of the amniotic fluid volume. This means these measurements are estimates rather than an actual true measurement of the fluid present. There is also no clear definition of normal/abnormal measurements of fluid volume across gestation for care providers to base diagnosis of high or low levels on.
What Is Oligohydramnios?
Oligohydramnios is diagnosed when there is not enough amniotic fluid volume surrounding the baby. AFI measurement is <5 and SDP is <2cm. Approximately 4% of women are diagnosed with oligohydramnios.
In most cases, the cause of oligohydramnios is unknown. As pregnancy progresses past 37 weeks, the fluid levels naturally decline. Each mother-baby unit is unique, and some may simply have lower levels than what is considered average. Maternal hydration levels appear to have an impact as well, so a scan may show lower than usual levels one day, but be normal a few days later, depending on how much fluid the mother has taken.
If the amniotic sac has ruptured and developed a slow leak, this may also contribute to lower levels of fluid. A review of research literature shows that in a health full term pregnancy, oligohydramnios isn’t linked to complications but does increase the risk of interventions.
In certain situations, there are problems with the baby’s kidneys or placental function. This is more likely to occur with medical conditions such as pre-eclampsia. Babies in these situations are usually small for dates and can be easily felt through the mother’s abdomen.
Complications of oligohydramnios depend on the cause and when the condition is diagnosed. The earlier oligohydramnios occurs in pregnancy, the more severe the complications:
- Impaired fetal lung development
- Premature birth
- Club feet (talipes)
- Restricted fetal growth
- Breech presentation
- Cord compression and fetal distress
In most cases, induction is recommended for oligohydramnios, regardless of whether the cause is known or not. There is an increased chance the placenta is not functioning properly if the cause is due to a condition rather than normal decline in late pregnancy. Induction does have risks however, as babies who may be experiencing less placental support will be more likely to suffer distress during an induction, or even spontaneous labour.
Other management methods include an injection of fluid prior to labour (via amniocentesis, which is a needle inserted into the uterus through the abdomen). During labour, fluid can be inserted into the uterus by a catheter, known as amni-infusion. This helps keep the umbilical cord from being compressed. Women who have been given a diagnosis may need rehydration via oral or IV fluids, which often improves amniotic fluid levels.
What Is Polyhydramnios?
Polyhydramnios is diagnosed when there is too much fluid surrounding the baby. SDP measurement is >8cm or AFI measurement is >25cm. About 2% of women experience polyhydramnios and it is usually not known what causes the increase in fluid.
Certain factors which are associated with polyhydramnios are:
- Maternal diabetes, including gestational diabetes.
- Twin pregnancy
- Build up of fluid in certain areas of the baby’s body, called hydrops fetalis
- Blockage in the baby’s gut preventing them from absorbing amniotic fluid (gut atresia)
- Baby producing too much urine
- Baby not swallowing enough fluid
- Genetic problems with the baby
- Overgrowth of placental blood vessels (chorioandioma).
Complications of polyhydramnios tend to be the increased risk of preterm labour, due to the pressure being placed on the uterus with the extra fluid. Babies are also more likely to move into positions which aren’t ideal for birth. This could mean the umbilical cord can slip in front of the baby’s head, causing cord prolapse, which is a medical emergency. Placental abruption is another medical emergency, which may occur with a sudden increase in fluid volume.
While most of these complications are rare, it's likely your care provider will want to do some investigation to see if there is an underlying cause of polyhydramnios. In about 60% of cases, there is no known cause.
Depending on the cause and gestation, there are a number of treatment options. In some situations, excess amniotic fluid can be removed via a needle (amniodrainage) which can reduce the risk of premature labour. If there is a more urgent need to have the baby born sooner, your care provider may rupture your membranes, while holding the baby in place. This controlled induction is carefully managed to avoid cord prolapse, but does require quick access to a theatre in case a c-section becomes necessary.
The amniotic fluid plays an important role in the health and development of your growing baby. If your care provider believes there is an issue with amniotic fluid levels, a number of scans may be necessary to determine if the volume is normal for you or there is an underlying problem. As with any intervention, it’s important you are provided with all the necessary information to make an informed decision about your care.
Recommended Reading: Why All Inductions Are Not The Same – 5 Induction Methods.