Meconium aspiration syndrome (MAS) occurs when a newborn baby breathes in fluids that cause breathing difficulties. The symptoms can be distressing but are usually temporary.

MAS is a rare condition that typically affects a small number of full-term or post-term newborns. Some babies will require breathing support, including an oxygen mask and mechanical ventilation.

MAS can sometimes lead to complications, such as pneumonia. However, most babies will recover and experience no long-term complications.

In this article, we examine MAS and its causes. We also look at how to treat MAS and the possible complications.

Close up of baby with meconium aspiration syndrome.Share on Pinterest
Image credit: Jill Lehmann Photography/Getty Images

MAS occurs when a newborn breathes in a mixture of amniotic fluid and meconium.

Meconium is a dark green sticky substance comprising materials that the fetus ingested while in the uterus. Meconium passes out of the newborn’s body as their first stool. Amniotic fluid surrounds the fetus in the uterus.

If a baby breathes in amniotic fluid and meconium, these substances can reach their lungs, blocking the airways. The condition can become severe and cause part of the lung to collapse. The resulting signs include difficulty breathing and a bluish tint to the skin.

The condition usually affects full-term newborns born at 37–41 weeks of pregnancy who are small for their gestational age or post-term newborns born after 42 weeks of pregnancy.

MAS also increases the likelihood of serious infections, such as pneumonia, which can be particularly severe in a newborn, as their immune system is not yet mature.

MAS is a rare complication during childbirth. A study in the International Journal of Pediatrics estimated that about 0.18% of full-term newborns could experience this condition.

A newborn gets MAS as a result of breathing in meconium and amniotic fluid.

Most newborns pass meconium shortly after birth. However, it is possible — particularly in post-term pregnancies — for a baby to pass meconium while still in the uterus.

Normal labor contractions or other factors, such as infection or low blood oxygen levels, may stress the baby.

Stress can lead a baby to take forceful breaths, causing fluids from the uterus to rush into their lungs. This can happen before, during, or immediately after birth.

MAS is most common in full-term underweight newborns or post-term newborns. However, other risk factors include:

  • diabetes during pregnancy
  • high blood pressure during pregnancy
  • fetal distress
  • fetal hypoxia (low oxygen level)
  • smoking during pregnancy

Possible signs of MAS include:

  • rapid breathing
  • bluish skin
  • pulling of the chest wall
  • labored breathing
  • grunting while breathing
  • limp body

Long-term exposure to meconium in the amniotic fluid can also cause a yellowing of the skin and nails.

Many newborns with MAS will not experience long-term complications. However, if meconium reaches the lungs, it can cause breathing difficulties and restrict the amount of oxygen available to the body.

About 30–50% of newborns with MAS require breathing support, such as a mechanical ventilator.

MAS can also trap air in the lungs. If the lungs expand too much, they can rupture and collapse. Severe MAS can also lead to infections, such as pneumonia.

Some newborns may develop pulmonary hypertension. This condition lowers the amount of blood flowing into the lungs, leading to a lack of oxygen in the body.

Most babies will recover from complications, but, rarely, they can be fatal.

The treatment for MAS will depend on the amount of meconium that the newborn breathes in, the duration of exposure, and the newborn’s symptoms and overall health.

Doctors may use suction devices to clear the upper airways, including the throat and mouth. Some newborns may also require clearing of the lower airways, which doctors will do by placing a tube down the windpipe (trachea).

Some babies may need to receive additional oxygen via a face mask. A mechanical ventilator may also be necessary to support their breathing and keep the lungs inflated.

If there are signs of a bacterial infection, a doctor may prescribe antibiotics.

It is not always possible to prevent MAS. However, in post-term pregnancies, doctors may recommend inducing labor to reduce the risk of the condition.

Doctors will usually check the amniotic fluid at birth and look for meconium. If meconium is present in the amniotic fluid, the doctor will check for signs of MAS.

For example, they may listen to the newborn’s breathing through a stethoscope. Abnormal breathing patterns or grunting sounds are indicators for MAS.

The doctor may also use a laryngoscopy, which is an examination of the throat, including the vocal cords.

To confirm a diagnosis, a doctor might order a chest X-ray or blood gas test.

A pregnant person should contact a doctor immediately if their water breaks and there is a dark green substance in the fluid. This substance may be meconium, in which case, medical attention may be necessary to prevent complications.

MAS causes short-term complications, including difficulties breathing or changes to the color of the skin. The newborn may require breathing support during this period.

More severe complications can occur, such as infection or low blood oxygen levels. In a small number of newborns with MAS, complications can be fatal.

However, most babies will recover within a few days and experience no long-term difficulties.