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What is Amniotic Fluid?

Almost every pregnant woman has the same fear: that their bag of water will break in some public place and leave a huge, embarrassing trail of fluid for all to see. Luckily that isn’t the case for many, but here’s a look at what amniotic fluid is and why your baby needs it.

Amniotic fluid is the liquid that surrounds your growing baby. It starts to form just a few weeks after your pregnancy begins. In the beginning, we aren’t completely sure where it comes from, but it is likely that the majority of the fluid comes from the water content of your blood that crosses the placenta.

By the second trimester, your baby is the main producer of this fluid, with two main sources secreting it: the lungs and the kidneys. The kidneys are actually the main source. That’s right, your baby swims in his or her own urine (thankfully, this is a sterile fluid)! Your baby’s kidneys actually produce between 1.5 and 5 cups of urine—also known as amniotic fluid—every day.

The amount of amniotic fluid around your baby is usually quite stable, with normal ranges that we can measure on ultrasound. Since your baby makes so much fluid every day, you might be wondering where it all goes. Your baby actually breathes and swallows amniotic fluid regularly, starting after 18 weeks. Yet another realization: your baby’s first drink (and this was yours, too!) was his or her own urine.

So why do we need amniotic fluid, other than to swim around in, before we are born? It has several important functions. If we fall, it provides a protective cushion. Since your baby first breathes and swallow this fluid, it helps the development of his or her lungs and gastrointestinal tract. The expanding fluid also helps put pressure on the uterus to grow, and it also gives room for your baby to practice movements and exercise those growing muscles.

As physicians, we have found other ways to make amniotic fluid useful, too. Fetal cells can be found in this fluid, which is why your obstetrician will sometimes use this fluid for genetic testing. An example of this is an amniocentesis when a sterile needle is passed through the uterus into the amniotic sac to obtain a small sample of this fluid. It can then be used to identify certain genetic disorders.


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  • Obstetrics: Normal and Problem Pregnancies
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  • Fetal Physiology.
    The American Congress of Obstetricians and Gynecologists
  • Your Pregnancy and Birth
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