Amniotic Fluid: Function, Volume and Abnormalities
Amniotic fluid is the protective liquid surrounding the unborn baby inside the amniotic sac. It plays a vital role in fetal development throughout pregnancy.
Things worth knowing about "Amniotic fluid"
Amniotic fluid is the protective liquid surrounding the unborn baby inside the amniotic sac. It plays a vital role in fetal development throughout pregnancy.
What is Amniotic Fluid?
Amniotic fluid is a clear to slightly yellowish liquid that surrounds the unborn baby throughout pregnancy within the amniotic sac. It is essential for the healthy development of the fetus and serves multiple critical physiological functions.
Composition and Formation
In the early stages of pregnancy, amniotic fluid is primarily produced by the mother. From approximately week 20 of pregnancy, the majority of the fluid originates from the fetal urine. The baby continuously swallows amniotic fluid, processes it through the kidneys and lungs, and excretes it back – forming a natural circulatory cycle.
Amniotic fluid is composed of:
- Water and electrolytes
- Proteins and carbohydrates
- Fatty acids and hormones
- Fetal skin cells and lanugo hair
- Antibodies that support immune defense
Functions of Amniotic Fluid
Amniotic fluid performs numerous vital functions for the unborn child:
- Cushioning and protection: It protects the baby from physical shocks and external pressure.
- Temperature regulation: It maintains a stable and warm environment for the baby.
- Freedom of movement: It allows the baby to move, which is essential for the development of muscles and the skeletal system.
- Lung development: The baby breathes in and out amniotic fluid, which promotes the maturation of the fetal lungs.
- Infection protection: Antibodies within the fluid help protect the baby from infections.
- Umbilical cord protection: It prevents compression of the umbilical cord.
Volume of Amniotic Fluid During Pregnancy
The amount of amniotic fluid changes throughout the course of pregnancy:
- 16th week of pregnancy: approximately 200 ml
- Around week 34: maximum volume of approximately 800–1,000 ml
- Toward the end of pregnancy: slight decrease to approximately 600–800 ml
Abnormalities in Amniotic Fluid Volume
Polyhydramnios (too much amniotic fluid)
Polyhydramnios refers to an excessive accumulation of amniotic fluid, typically above 2,000 ml. Possible causes include:
- Gestational diabetes
- Fetal malformations of the digestive tract or nervous system
- Multiple pregnancies (twins, triplets)
- Infections during pregnancy
Oligohydramnios (too little amniotic fluid)
Oligohydramnios occurs when the volume of amniotic fluid falls significantly below normal levels. Possible causes include:
- Fetal kidney malformations
- Placental insufficiency
- Post-term pregnancy
- Premature rupture of membranes
Rupture of Membranes
At the end of pregnancy or at the onset of labor, the amniotic sac breaks – this is called the rupture of membranes or water breaking. The amniotic fluid then flows out through the vagina. A premature rupture of membranes before week 37 of pregnancy requires immediate medical evaluation.
Diagnostic Use of Amniotic Fluid
Amniotic fluid can also be used for diagnostic purposes. In a procedure called amniocentesis, a small sample of amniotic fluid is extracted to analyze fetal cells. This test can detect:
- Chromosomal abnormalities (e.g., Trisomy 21 / Down syndrome)
- Genetic disorders
- Infections
- Fetal lung maturity
When to Seek Medical Advice
The following signs require prompt medical attention:
- Fluid leaking from the vagina (possible rupture of membranes)
- Changes in amniotic fluid color (greenish or brownish fluid may indicate meconium passage)
- Severe abdominal pain or pressure
- Reduced fetal movement
References
- Moore, K.L.; Persaud, T.V.N.; Torchia, M.G.: The Developing Human – Clinically Oriented Embryology. 11th edition. Elsevier, 2019.
- Magann, E.F. et al.: Amniotic fluid and the clinical relevance of the sonographically estimated amniotic fluid volume. Journal of Ultrasound in Medicine, 2011. Available at: https://pubmed.ncbi.nlm.nih.gov/21876098/
- World Health Organization (WHO): WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience. WHO Press, Geneva, 2016. Available at: https://www.who.int/publications/i/item/9789241549912
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