During pregnancy, the baby grows inside a fluid filled 'bag' which sits inside the uterus. The bag is known as the amniotic sac and the clear, straw-coloured liquid inside the sac is the amniotic fluid.
The amniotic fluid serves several main functions, which are to cushion the baby from knocks and bumps; protect against infection; nourish the baby and allow its lungs and digestive system to grow and mature; and to keep the baby at a consistently warm temperature.
It also allows the baby to move, kick and (in early pregnancy) somersault without restriction.
Amniotic fluid begins to fill within the forming amniotic sac around two weeks after conception. To begin with, it is mainly water. By 12 weeks of pregnancy, the fluid is filled with nutrients such as carbohydrates and proteins.
As the baby grows, the amniotic fluid increases and reaches its maximum level at around 36 weeks of pregnancy, when the woman will be carrying almost two pints of fluid.
Throughout pregnancy, the baby will swallow the amniotic fluid, and will pass it back out as urine. It is also inhaled by the baby, which helps to mature the baby's lungs.
Because amniotic fluid contains fetal cells, samples of it can be taken to determine the presence of a range of medical conditions, including Down's syndrome. This procedure is known as an amniocentesis.