The amniotic sac is a bag of fluid in the uterus, where the unborn baby develops. It’s filled with a colourless fluid – mainly made of water – that helps to cushion the foetus and provides fluids which enable the baby to breathe and swallow. The fluid also guards against infection to either the foetus or the uterus. Amniotic fluid plays a vital role in the development of internal organs, such as the lungs and kidneys; it also maintains a constant temperature. The amniotic sac starts to form and fill with fluid within days of conception.