The origin and composition of amniotic fluid changes over the course of pregnancy, and to some extent its many roles do as well.
In the first 20 weeks of gestation the fluid is direct exudates of the maternal plasma and fetal tissue. The fetal skin is not keratinized (meaning not covered with mostly-watertight dead cells) and the amniotic fluid has an electrolytic and proteinacous composition very similar to plasma and fetal fluids. The fluid just seeps out from everywhere.
After about 20 weeks, not only is the skin keratinizing, but also the urinary and gastrointestinal systems are maturing. And so the composition of the fluid changes. It becomes (mostly) fetal urine. The fetus swallows it. The fetus `breathes' it in and out. And the fetus pees more of it out again.
And so, in addition to being a good cushion and swimming pool for the baby to develop in, it also helps distribute nutrients early on, and later helps the lungs, diaphragm, and GI system develop properly.
Interestingly and intuitively, improper development of the kidneys or blockage of the ureters/bladder/urethra can lead to inadequate amounts of fluid, and blockage of the upper GI tract or lack of development of neural structures, and thus lack of the swallow reflex, can lead to excessive fluid.
The fluid is contained within the amnion. This is the sac that would tear open when a mom-to-be's water breaks sending forth that gush of amniotic fluid that often means a baby is on the way.
BTW, it's really the amniotic sac that provides a barrier to infections, not the fluid.
Finally, those burning questions answered:
- It is pretty clear, very slightly straw-colored by itself. But the cheesy substance covering the mature fetus (vernix) usually makes a lot of flocculent bits in it. And if the baby poops (meconium) before it's born, it can range from green-tinged to looking like split-pea soup.
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It smells pretty innocuous. A little like fresh blood.
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It feels mostly very warm. The first time that I was hit by a flood of amniotic fluid (during a c-section) I almost passed out. Now I just try hard not to get hit by it.
- It tastes kinda salty. If do you get hit in the face by it, you find yourself pursing your lips until you can wash your face well. Enough will get in that it tastes slightly salty, though. And then you try harder not to get hit in the face with it ever again.
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