Amniotic fluid is the fluid that surrounds the foetus (unborn baby) in the womb during the course of the pregnancy. It is approximately 98% water. In common terms, the amniotic fluid is what people refer to when they say their “water broke.” The membrane or sac that holds the fluid is called an amniotic sac or the “bag of waters.”
The amniotic fluid volume increases linearly throughout pregnancy to a maximum of 400 to 1200 mL at 34 to 38 weeks of gestation, at which point the amniotic fluid volume (AFV) levels off (at approximately 400 mL) and remains constant until term.
Let’s find out more about the amniotic fluid.
Why is The Amniotic Fluid Important?
Amniotic fluid is a workhorse that:
Allows for foetal movements within the uterus (womb) which helps in the foetal muscle and bone development because of which it is used as a marker to assess foetal health and development.
Protects the foetus from injury/impact as it acts as a cushion. It also prevents the compression of the umbilical cord (the tube that carries nutrition and oxygen from the placenta to the foetus) against the uterine wall.
Aids in lung and digestive system development. As the foetus breathes and swallows the amniotic fluid, the muscles involved in these actions are strengthened.
Prevents infections in the intra-amniotic environment.
Provides a consistent temperature for the development of the foetus and also prevents heat loss.
What Happens When There is Too Little or Too Much of The Amniotic Fluid?
Too little of the fluid, a condition known as oligohydramnios, is when the volume of amniotic fluid decreases. A condition called polyhydramnios occurs when the volume of the amniotic fluid increases relative to the gestational age.
Causes of Oligohydramnios
Premature rupture of membranes (PROM).
Maternal factors such as diabetes, hypertension, systemic lupus erythematosus (SLE), pre-eclampsia, obesity, dehydration, and infections.
Foetal anomalies, particularly those that decrease urine production (kidney abnormalities).
Placental insufficiencies (such as placental abruption).
Chromosomal abnormalities (such as aneuploidy).
Intrauterine death or growth restriction.
Drugs such as angiotensin-converting enzyme (ACE) inhibitors, non-steroidal anti-inflammatory drugs (NSAIDs), etc.
Causes of Polyhydramnios
Foetal malformations (gastrointestinal or urinary tract obstruction).
Maternal factors such as diabetes, Rh incompatibility, multiple (twin) gestation, and infections.
Foetal anaemia.
Placental tumours (eg: sacrococcygeal teratoma, placental chorioangioma, etc.).
What are the Complications of Oligohydramnios and Polyhydramnios?
If the pregnancy is complicated by oligohydramnios, the foetus might suffer from any one of the many complications listed below:
1. Foetal malformations such as the oligohydramnios sequence or Potter’s syndrome. It is characterized by:
Pulmonary hypoplasia: Underdeveloped lungs leading to respiratory distress after birth.
Limb contractures (club feet and joint contractures).
Potter’s facies (low set ears, retrognathia, and hypertelorism).
Renal agenesis (absence of one or both kidneys) or intrauterine growth retardation (IUGR).
2. Stillbirth
3. Preterm birth
4. Neonatal death
5. If the baby is born, it is likely to be delivered through a C-section or be a low birth weight baby that will require neonatal intensive care unit (NICU) care and support.
If the pregnancy is complicated by polyhydramnios, it might lead to:
1. Preterm labour due to premature contractions.
2. Premature rupture of membranes (PROM).
3. Placental abruption (placenta peels away from the uterine wall even before the foetus is delivered).
4. Umbilical cord prolapse.
5. Postpartum haemorrhage (heavy bleeding post-delivery) and uterine atony.
6. Foetal death or stillbirth.
How Are The Complications Diagnosed?
Ultrasound Diagnosis: The vertical measurement of the deepest pocket of amniotic fluid free of foetal parts is measured. The sum of the vertical depth of fluid measured in each of the 4 quadrants of the uterus is termed the amniotic fluid index (AFI). The normal AFI ranges from > 5 to < 24 cm; values ≤ 5 cm indicate oligohydramnios and >24 cm indicates polyhydramnios.
Apart from AFI measurement, a comprehensive ultrasonographic examination is also done to check for fetal malformations, placental insufficiencies, etc.
How Are The Complications Managed?
In the case of oligohydramnios:
Serial monitoring via ultrasonography (USG) - done at least once every 4 weeks (every 2 weeks if intrauterine growth restriction is suspected or detected). AFI should be measured at least once a week. In cases of PROM, a lung assessment can be useful in predicting and preventing pulmonary hypoplasia.
Non-stress testing (NST) and biophysical profile of the foetus.
The ideal time for delivery can vary from patient to patient. It is often left to your treating doctor’s discretion who will decide based on patient characteristics and foetal complications.
In the case of polyhydramnios:
Prenatal monitoring depends on the severity of the condition, based on AFI. If AFI is ≥ 30 cm (which increases the risk of foetal death), it is suggested to begin as early as 32 weeks or whenever it is diagnosed; it should also include NST at least once a week.
Ultrasonography should be done every 4 weeks to check for anatomical abnormalities.
Often delivery at about 39 weeks is planned and the mode of delivery depends on maternal and foetal indications (such as presenting part of the foetus, cervical dilatation, etc.).
Drainage of excess amniotic fluid is only reserved for patients who present with preterm labour or premature rupture of membranes.
Conditions contributing to polyhydramnios (eg: maternal diabetes) should be controlled.
If you are concerned about the levels of amniotic fluid during pregnancy, consult your doctor immediately.
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