The amniotic sac is a bag of fluid inside a woman's womb (uterus) where the unborn baby develops and grows.
It's sometimes called the "membranes" because the sac is made of 2 membranes called the amnion and the chorion.
The amniotic sac is filled with clear, pale, straw-coloured fluid in which the unborn baby floats and moves.
The amniotic fluid helps to cushion the baby from bumps and injury, and maintains constant temperature for the baby.
It also helps your baby's lungs, digestive system and musculoskeletal system to develop.
The amniotic sac starts to form and fill with fluid within days of a woman conceiving.
Amniotic fluid is mainly water. The unborn baby swallows the amniotic fluid and passes tiny amounts of urine into the fluid.
The amount of amniotic fluid increases gradually during pregnancy until about week 38, when it reduces slightly until the baby's born.
What happens when my waters break?
Before or during labour, the amniotic sac breaks and the fluid drains out through the vagina. This is commonly known as your waters breaking.
If your waters break before your labour starts, you'll notice either:
- a slow trickle from your vagina, or
- a sudden gush of water that you can't control
Phone your midwife or the hospital for advice.
If you plan to give birth in hospital or a midwife unit, you'll probably be advised to go in at once. Without amniotic fluid, your baby is no longer protected and is at risk of infection.
Always speak to your midwife or GP if you're concerned about any aspect of your health when you're pregnant.
You can also call NHS 111.
Page last reviewed: 26 September 2018
Next review due: 26 September 2021