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Meconium Aspiration Syndrome (MAS)

What Is Meconium?

Meconium is a newborn's first poop. This sticky, thick, dark green poop is made up of cells, protein, fats, and intestinal secretions, like bile. Babies typically pass meconium (mih-KOH-nee-em) in the first few hours and days after birth. But some babies pass meconium while still in the womb during late pregnancy.

What Is Meconium Aspiration Syndrome?

Meconium aspiration syndrome (MAS) happens when a newborn has trouble breathing because meconium got into the lungs.

Meconium can make it harder to breathe because it can:

  • clog the airways
  • irritate the airways and injure lung tissue
  • block surfactant, a fatty substance that helps open the lungs after birth

With treatment, most babies with meconium aspiration syndrome get better with no problems.

What Causes Meconium Aspiration Syndrome?

Meconium aspiration happens when a baby is stressed and gasps while still in the womb, or soon after delivery when taking those first breaths of air. When gasping, a baby may inhale amniotic fluid and any meconium in it.

Babies are more likely to pass meconium when:

Babies who are stressed by low oxygen levels or infections also may pass meconium before birth. When meconium gets in the amniotic fluid, there's a chance a baby will breathe (aspirate) it into the lungs before, during, or after birth. But most babies with meconium in the amniotic fluid will not get MAS.

What Are the Signs & Symptoms of Meconium Aspiration Syndrome?

Before or at a baby's birth, doctors will notice one or more of these signs:

  • The amniotic fluid is meconium-stained (green).
  • The baby has meconium stains.
  • The baby has breathing problems or a slow heart rate.
  • The baby is limp.

How Is Meconium Aspiration Syndrome Diagnosed?

Doctors diagnose MAS in any baby with breathing problems who is born through meconium-stained fluid and has no other cause for the breathing problems. A chest X-ray can confirm the diagnosis. Doctors may do other tests to be sure nothing else causes the symptoms, such as a heart problem or pneumonia.

How Is Meconium Aspiration Syndrome Treated?

Most babies with MAS get medical care in a special care nursery or neonatal intensive care unit (NICU) and get oxygen, if needed.

A baby who gets extra oxygen but still struggles to breathe will get help from a breathing machine (ventilator). An infant with severe MAS may need more treatment, such as:

  • surfactant to help open lungs
  • inhaled nitric oxide. This gas is added to oxygen to open blood vessels and improve oxygen delivery.
  • extracorporeal membrane oxygenation. The ECMO machine, using a pump that works like the heart, pumps blood from the body through an artificial lung. Like a normal lung, it adds oxygen to the blood and removes carbon dioxide. Then the machine sends the blood back to the child.

Most babies with MAS get better within a few days or weeks, depending on how much meconium they inhaled.

Can Meconium Aspiration Syndrome Be Prevented?

If a woman goes past her due date, her doctor may recommend inducing labor to help prevent MAS. If a pregnant woman's water breaks and she sees dark green stains or streaks in the fluid, she should tell her doctor right away. This is a sign that meconium is in the amniotic fluid.

In the past, doctors sometimes did amnioinfusion (using saline to dilute meconium in the amniotic fluid) or suctioned meconium out of the baby's mouth and throat at birth. But these treatments weren't found to prevent MAS, so are not routinely done.

What Else Should I Know?

Most infants with meconium aspiration syndrome recover completely. Some babies may have a higher risk of lung infections and wheezing, particularly in their first year of life.

Date reviewed: January 2020