What Is Amniotic Fluid?

Medically Reviewed by Dany Paul Baby, MD on April 07, 2022
4 min read

Amniotic fluid is the liquid that surrounds and cushions your baby in the womb. It protects your baby and helps in its development. When your "water breaks," amniotic fluid leaks out and signals that your baby is on its way. 

Read on for more about amniotic fluid, where it comes from, and how it helps your baby. 

During pregnancy, your baby develops inside a bag or sac called the amnion or amniotic sac. This sac contains a clear liquid called amniotic fluid. Your baby floats in it while resting in the womb. Amniotic fluid protects and nourishes your baby. 

The amniotic sac starts forming 12 days after conception. As your baby starts to grow, the sac fills with amniotic fluid. In the early stage of pregnancy, the fluid contains mostly water from your body. However, its chemical makeup changes over the course of your pregnancy. 

Amniotic fluid is mainly made of water from your body. Its contents change, though, from early to late pregnancy. 

In the early stages of pregnancy, amniotic fluid contains 98% water and electrolytes or minerals. It is derived from fluids from your body cavity and serum or the liquid part of your blood. 

The remaining 2% includes:

  • Antibodies, proteins made by the immune system to fight infections 
  • Hormones, chemical messengers in the body
  • Nutrients like proteins, carbohydrates, lipids, and vitamins

After 20 weeks of pregnancy, amniotic fluid mostly consists of your baby’s urine. It also contains lung secretions expelled after your baby breathes in some of the amniotic fluid.

Amniotic fluid is typically colorless or slightly yellow. Sometimes, the fluid looks green or brown. This happens if the baby passes its first bowel movement in the womb, a process known as meconium. Babies usually have their first bowel movement after birth. 

If meconium enters the amniotic fluid, it can be harmful to the baby. The baby inhales some amniotic fluid in the womb. If the meconium enters the baby’s lungs through the amniotic fluid, it can cause serious breathing problems. This condition is called meconium aspiration syndrome. Babies who pass meconium in the womb may need treatment immediately after birth to prevent breathing problems.

Amniotic fluid functions include:

  • Providing space, water, and nutrients for your baby to grow 
  • Cushioning that protects your baby from sudden movements and trauma
  • Cushioning that protects the umbilical cord, which carries food and oxygen from your body to your baby, from being squeezed 
  • Maintaining a steady temperature for your baby 
  • Protecting your baby from infections and diseases with your own antibodies
  • Helping your baby’s lungs develop, as your baby breathes in and expels some amniotic fluid 
  • Helping your baby’s digestive system develop as your baby swallows amniotic fluid 
  • Helping your baby’s muscles, bones, and limbs develop as your baby floats and moves around in amniotic fluid 

The amount of amniotic fluid keeps increasing until 34 weeks of pregnancy. At that time, about 800 milliliters of amniotic fluid surround the baby. After that, the amount starts decreasing. At 40 weeks of pregnancy, there are about 600 milliliters.

Sometimes you may have too little or too much amniotic fluid. When you have too little amniotic fluid, it is called oligohydramnios. If you have too much fluid, it is called polyhydramnios. These conditions may cause problems during pregnancy, but most babies will still be born healthy.

Oligohydraminos. Oligohydramnios occurs when you have too little amniotic fluid. If you have less than 500 milliliters of amniotic fluid from the 32nd to 36th weeks of pregnancy, your doctor will diagnose oligohydramnios. It can occur in women with late pregnancy, injury, or other pregnancy-related problems.

Less amniotic fluid means fewer nutrients for your baby. The baby won’t be able to grow as expected, causing a condition known as intrauterine growth restriction. Oligohydramnios during the first 6 months of pregnancy can lead to serious problems. In the early weeks of pregnancy, oligohydramnios can lead to birth defects, miscarriage, premature birth, or stillbirth.

In the last stage of pregnancy, oligohydramnios can lead to slow growth. It can also lead to a squeezed umbilical cord, preventing the baby from getting enough food and oxygen.  Oligohydramnios also increases the risk of a cesarean section or surgical delivery of the baby. 

Polyhydraminos. Polyhydramnios is the opposite of Oligohydramnios. It is a rare condition that happens in 1% of pregnancies. It occurs when too much amniotic fluid collects in the womb during pregnancy.  

This can occur if you have genetic problems, obstructions in your digestive system, or bone or muscle problems. It can also occur with gestational diabetes, which is diabetes that occurs during pregnancy. It is also seen in women who have twins or triplets.

Usually, polyhydramnios isn’t harmful, but it may cause serious complications. These include:

  • Premature or pre-term labor
  • Premature birth
  • Excessive growth of your baby
  • The placenta getting detached from the womb before birth
  • Hemorrhaging, heavy bleeding after birth
  • The umbilical cord falling out of the vagina before the baby
  • Abnormal position of the baby in the womb
  • Stillbirth

Doctors use your amniotic fluid to monitor the progress of your pregnancy. They also use it to predict the baby’s condition. Doctors measure the amount of amniotic fluid using an ultrasound. They use methods like amniotic fluid index (AFI) assessment or maximum vertical pocket (MVP) measurement. 

Doctors also use amniocentesis, which involves the collection of amniotic fluid from the womb using a needle. This diagnostic test determines if your baby has genetic disorders, birth defects, or other health conditions.

If you’re pregnant and notice fluid leaking from your vagina, contact your doctor immediately. Your doctor will see if you’re not gaining weight or your baby isn’t growing as expected. These can be signs of oligohydramnios.