"The amnion grows and begins to fill, mainly with water, around two weeks after fertilisation. After a further 10 weeks the liquid contains proteins, carbohydrates, lipids and phospholipids, urea and electrolytes, all which aid in the growth of the fetus. Most of the amniotic fluid comes from the baby's urine. By the second trimester the fetus can breathe in the water, allowing normal growth and the development of lungs and the gastrointestinal tract."
Quoted from wikipedia. Amniotic sac - Wikipedia, the free encyclopedia
