Amniotic Sac

What is the amniotic sac?

The amniotic sac, also known as the “bag of water” or the membranes, surrounds a fetus during pregnancy. The amniotic sac is a thin, tough, transparent pair of membranes that surround the amniotic fluid, placenta and umbilical cord. The main job of the amniotic sac is to protect the fetus from infection or injury. As the fetus develops, the amniotic sac grows. If or when the amniotic sac ruptures, called your “water breaking,” it can be a gush of fluid or a trickle. It’s important to immediately notify your pregnancy care provider if you think your water has broken. It can be one of the common signs of labor.

When does the amniotic sac form?

The amniotic sac forms about 12 days after a sperm fertilizes an egg (conception).

What does the amniotic sac do?

The main job of the amniotic sac is to protect the fetus. The sac holds amniotic fluid, which helps cushion the fetus from bumps and regulate its body temperature. It also holds the umbilical cord and the placenta, which are critical organs for supplying the fetus with nutrients and removing waste.

Without an amniotic sac, a fetus wouldn’t have a protective environment to grow and move around in. It’s critical that the amniotic sac doesn’t break open too soon — ideally, it doesn’t rupture until you reach 40 weeks of pregnancy.

How does the amniotic sac break?

When the amniotic sac breaks open, your healthcare provider typically refers to it as your membranes rupturing (or water breaking). You can sometimes feel amniotic fluid leaking or gushing out of your vagina. Several factors lead to your amniotic sac breaking. Some of them are:

• Hormones: Your body releases prostaglandins as labor begins. This hormonal change can weaken the sac and cause it to rupture.
• Pressure from contractions: Contractions are a tightening and releasing of the muscles in your uterus. This can put pressure on the amniotic sac and wear it down.
• Fetal movement: If the fetus is head down in your pelvis and moving frequently, this can be a factor in the sac weakening and eventually tearing.
• Weakening sac: The sac weakens due to cellular changes that happen naturally as the fetus gets closer to delivery.

Pregnancy care providers prefer that your amniotic sac stays sealed until you reach 37 weeks or more. This gives the fetus the best chance at a healthy delivery.

When does the amniotic sac break?

The amniotic sac breaks at the beginning of or during labor. For some people, it happens spontaneously (without warning), which means your body naturally does it.

But for some, their pregnancy care provider has to rupture the amniotic sac in a procedure called an amniotomy (or artificial rupture of membranes) during labor. This involves your provider using a hook-like device to puncture the sac. When this happens, it’s most often to speed up labor in someone who’s already in labor.

Where is the amniotic sac?

The amniotic sac is inside your uterus during pregnancy.

What does the amniotic sac look like?

The amniotic sac is made up of two layers of membranes called the chorion and the amnion. The amniotic sac is translucent, and you can almost see through it.

How much fluid is inside the amniotic sac?

It depends on how far you are in pregnancy. Your amniotic sac contains the most amniotic fluid around 34 weeks of pregnancy, but then begins to decrease as you near your due date. At its peak, there is a little less than 1 liter of fluid in the amniotic sac.

What are the health conditions that could affect the amniotic sac?

There may be times you hear about the amniotic sac during pregnancy. It could be because you have a health condition that affects your amniotic sac or because your provider needs to check on the health of the fetus.

Some of those conditions or situations include:

• Chorioamnionitis: This is an infection in the membrane layers of your amniotic sac (the amnion and the chorion).
• Premature rupture of membranes (PROM): This is when your amniotic sac breaks before labor begins. Once your water breaks, your risk for infection increases.
• Amniotomy: This is when your healthcare provider uses an amnihook to break your amniotic sac. Rupturing the sac may help speed up labor.
• Amniocentesis: This is a procedure that healthcare providers use to diagnose genetic conditions in a fetus before birth. It involves placing a needle through the amniotic sac to remove amniotic fluid.

How are conditions of the amniotic sac treated?

Treatment methods for conditions relating to the amniotic sac mainly depend on how far along you are in pregnancy and if labor has begun. Your healthcare provider can prescribe antibiotics to treat infections like chorioamnionitis. But for a condition like premature rupture of the membranes, delivery is usually the safest treatment. If you’re fewer than 37 weeks pregnant, your provider may give you medications to help develop the fetus’s lungs before delivery.

Can a baby survive a ruptured amniotic sac?

Yes. But it also depends on when the sac ruptures. It’s typically OK if the sac ruptures after 37 weeks of pregnancy because the fetus’s lungs are strong enough that they can breathe on their own at birth. But if the sac ruptures before 37 weeks, the fetus is more at risk for complications like difficulty breathing or trouble regulating their body temperature.

Can a baby be born inside the amniotic sac?

Yes. This is a rare event called an en caul birth. When a baby is born inside an intact amniotic sac, it almost looks like they’re born inside a bubble. If this happens, your healthcare provider will use medical tools to cut an opening in the sac to remove your baby.

What happens to the amniotic sac after birth?

The amniotic sac comes out with the placenta in the afterbirth, typically within 30 minutes after delivery. Once it comes out, your provider throws it away as medical waste or sends it to the lab for evaluation if indicated.