What is the amniotic sac

Amniotic sac, or the bag of water, is the membrane around the amniotic cavity, containing the colorless or straw-colored fluid (amniotic fluid) that surrounds and supports the developing baby within the uterus [1]. It is pushed out of the body along with the placenta and umbilical cord after delivering the baby.

Amniotic sac structure

Shortly after conception, the amniotic sac begins to form, gradually filling with water (amniotic fluid) in preparation for housing the growing embryo [1]. There are two membranes forming the amniotic sac:


It is the outer membrane of the sac, beginning from the placenta and chorionic villi. It is through this membrane that oxygen and all the necessary nutrients reach the fetus, ensuring its proper growth [2].


The inner membrane of the amniotic sac, the amnion is the layer directly in contact with the amniotic fluid and the fetus [3]. It fuses with the chorion layer around the 14th week of pregnancy [4].

Amniotic Sac

Amniotic Sac

Functions of the amniotic sac

It is one of the primary components of the fetal life support system, helping to keep the baby separated from the mother within the uterus throughout the gestation [5]. Its principal function is to keep the baby healthy, safe from any external bumps and pressure, protected by the cushiony layer of amniotic fluid [6].

There is ample space within the amniotic sac for the growing baby to move around, swim and kick all day so his limbs and muscle tone can develop properly [7].

Amniotic Sac Ultrasound

Amniotic Sac Ultrasound

Amniotic sac in a twin pregnancy

Both in the cases of identical and non-identical twins, each baby usually has his own amniotic sac and umbilical cord so they can develop properly and individually [7]. However, in some rare cases, identical twins may share the same sac along with the same placenta (monochorionic-monoamniotic twins) [8].

The amniotic sac at the time of birth

Once the pregnancy reaches full term, and it is time for the labor to start, the amniotic sac breaks (the waters breaking), releasing the fluid through the vagina [9]. Call your doctor immediately if you notice a leakage, as there is an increased risk of several infections and complications, such as group B strep [10], if the baby is not delivered soon after the waters break [1].

Caul birth

Caul or “en caul” birth occurs when the baby is born while still within the amniotic sac. Though extremely rare, caul birth does not pose any threats to the baby or the mother. The newborn remains well and healthy when the membranes are removed post delivery [13].

Baby Born in Amniotic Sac

Baby Born in Amniotic Sac

Artificial rupture of the membranes (Amniotomy)

Amniotomy is the surgical procedure for manually rupturing the amniotic membrane to release the amniotic fluid, often for inducing labor or shortening its duration [11].

It might also be necessary for internal fetal monitoring, visual observation or collecting an amniotic fluid sample for examination [12].

Complications associated with the amniotic sac

Premature rupture of membranes (PROM)

A common pregnancy complication, PROM occurs due to the amniotic membranes breaking open before term, causing the fluid to leak or gush out [14]. Sometimes, the amniotic sac may repair itself to seal minor tears high in the sac [15]. A rupture later in the third trimester is often followed by labor and delivery to reduce the risks of any life-threatening infections and problems. The mother may be kept in close observation to evaluate the fetal lung maturity to make sure if the baby will be able to survive on his own after birth [16].

Blighted ovum

A blighted ovum is a kind of miscarriage where the egg implants itself in the uterus, but the embryo fails to develop [17]. The gestational sac develops with the yolk sac and the placenta also beginning to grow, but an ultrasound shows an empty sac as there is no embryo [18].

Amniotic band syndrome

Amniotic band syndrome or ABS can occur when thin strips tear away from the amnion and get entangled with the baby or wrap around his limbs, restricting the blood flow and hampering proper growth. [19].

Amniotic sac prolapse

Sometimes, the amniotic sac may bulge out through an opening in the cervix before the waters break, and labor begins [20]. Close monitoring and bed rest may be necessary so the pregnancy can be carried to term.

Chorioamniotic separation

It occurs when the chorion and amnion membranes of the amniotic sac separates from each other within the uterus, increasing the risk of various complications and birth defects [21].

  1. http://www.nhs.uk/chq/Pages/2310.aspx?CategoryID=54
  2. http://radiopaedia.org/articles/chorion
  3. http://www.medicinenet.com/script/main/art.asp?articlekey=2225
  4. http://radiopaedia.org/articles/amnion
  5. http://www.medscape.com/viewarticle/773578_2
  6. http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000130&ptid=17
  7. http://americanpregnancy.org/while-pregnant/fetal-life-support-system/
  8. http://www.chw.org/medical-care/fetal-concerns-center/conditions/infant-complications/birth-defects-in-monochorionic-twin/
  9. http://www.babycentre.co.uk/a568740/low-amniotic-fluid
  10. http://www.acog.org/Patients/FAQs/Group-B-Strep-and-Pregnancy
  11. http://apps.who.int/rhl/pregnancy_childbirth/childbirth/routine_care/CD006167_Wolombyj_com/en/
  12. http://emedicine.medscape.com/article/1997932-overview
  13. http://www.whattoexpect.com/wom/pregnancy/0226/how-a-baby-can-be-born-with-the-amniotic-sac-intact.aspx
  14. http://emedicine.medscape.com/article/261137-overview
  15. http://www.webmd.com/baby/tc/preterm-premature-rupture-of-membranes-pprom-topic-overview
  16. http://www.healthline.com/health/pregnancy/premature-rupture-tests#Overview1
  17. http://www.webmd.com/baby/guide/blighted-ovum
  18. http://americanpregnancy.org/pregnancy-complications/blighted-ovum/
  19. https://www.ucsfbenioffchildrens.org/conditions/amniotic_band_syndrome/
  20. http://pubs.rsna.org/doi/abs/10.1148/radiology.140.2.7255724?journalCode=radiology
  21. http://radiopaedia.org/articles/chorioamniotic-separation [/ref]