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What Is an Amniotic Sac?

The amniotic sac holds the amniotic fluid and the fetus during pregnancy.
The amniotic sac ruptures as pregnancy comes to a close.
Fraternal twins almost always have separated amniotic sacs, but sharing a sac can vary in identical twins.
It is occasionally possible to stop labor or to repair tiny injuries to the amniotic sac.
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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 17 December 2014
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The amniotic sac is a protective set of membranous tissues that form in female mammals and many other creatures during early pregnancy. The sac is also known as the bag or bag of waters, and it holds both amniotic fluid and the fetus. This provides an additional protective element for the fetus during development. As pregnancy comes to a close, the bag ruptures, or is occasionally intentionally ruptured to induce labor. Usually a baby is born free from the bag, but may occasionally be covered with some part of it, which is called a caul.

Several important functions exist for the amniotic sac. The fluid it holds provides some cushioning for the developing baby, and the fluid itself creates temperature control. It’s actually warmer by a degree or two than normal body temperature.

Fluid in the amniotic sac also allows babies to move very freely in the womb, especially in the second and early third trimester. Fetuses are, essentially, swimming, during this time period, and this explains how early movements felt by the mother can seem to occur in different places of “the belly” quite quickly. When people have water births, it is often the contention that birth into water is the most natural thing because the baby has been living in a “water state” for most of its short life.

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Given the importance of the amniotic sac, procedures like amniocentesis, where a needle is inserted into the sac, would seem very dangerous. In fact, in the majority of cases, amniocentesis is possible because the small puncture of the membrane won’t result in any form of leaking. The bag of fluids has self-sealing properties. This doesn’t mean that every amniocentesis is without risk, and sometimes this test has resulted in fetal injury or loss of pregnancy. Women can suffer from what is called premature rupture of the membranes, which may begin labor.

It is occasionally possible to stop labor or to repair tiny injuries to the amniotic sac. Women should be aware of any sensation that fluid is leaking, as this could indicate premature rupture. Alternately, when the bag breaks, at pregnancy term or earlier, some people feel a sudden rush of fluid that is very dramatic. Either scenario indicates that it is immediately time to contact a doctor, since both could mean labor is imminent.

In the standard single pregnancy, there is a sole amniotic sac. This is different in multiple births. Fraternal twins almost always have separated sacs, but sharing of a sac could vary in identical twins. Some are together in the same sac, and others each have their own. In higher multiples, it’s possible for a number of sacs and placentas to exist.

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