The amniotic sac starts to form and fill with amniotic fluid just days after a woman gets pregnant. At first, the amniotic fluid is made from the mother's plasma (the liquid part of her blood).
At first, the amniotic fluid is made mostly of water with electrolytes (salts). However, by the 12th to the 14th week of pregnancy, proteins, carbohydrates, lipids, phospholipids, and urea are all in the amniotic fluid. These things help the fetus grow.
As the fetus gets bigger, more amniotic fluid gets created. By the time the baby is born, there is about one liter of amniotic fluid in the amniotic sac.