Amniotic fluid (liquor amnii) is the liquid surrounding a foetus in the uterus of a pregnant woman. It is contained within the fibrous amniotic sac. A sample may be obtained in a process called amniocentesis by putting a needle through the skin of the lower abdomen and into the uterus and drawing off a small amount of amniotic fluid.
The amniotic fluid is created by the urine and faeces of the foetus, and by secretions from the placenta. The foetus is constantly swallowing and processing the fluid from about 15 weeks onwards, and it aids the growth and nutrition of the foetus.
It is normally a pale yellow colour, but may be darker if the foetus is distressed. The dark colouration may only be noticed at the beginning of labour when the waters break with the rupture of the amniotic sac in which the fluid and foetus are contained.
The volume of amniotic fluid steadily increases throughout pregnancy until about 36 weeks, after which it slowly decreases. At its peak, between 600 and 800 mLs of fluid are present.
The amniotic fluid acts as a cushion for the foetus, protecting it from external bumps, jarring and shocks. It also allows the foetus to move relatively freely, and allows equal growth in all directions. It contains protein, sugars, fats and electrolytes (sodium, potassium, salt etc.). Hormones and waste produced by the foetus are also present as these are excreted in the urine of the foetus.