NHS website - What is the amniotic sac?

What is the amniotic sac?

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 two membranes called the amnion and the chorion.

Amniotic fluid

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, as well as providing them with fluids that they can breathe and swallow.

It also helps the baby's musculoskeletal system to develop and maintains a constant temperature for the baby.

The amniotic sac starts to form and fill with fluid within days of a woman conceiving. Amniotic fluid is mainly water. From about week 10 of pregnancy and onwards, the unborn baby 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 is 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 will 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 will 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 are concerned about any aspect of your health when you are pregnant. You can also call NHS 111.

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