What Is Meconium Aspiration Syndrome?


Meconium Aspiration Syndrome (MAS) is a respiratory distress condition that affects babies shortly after birth and occurs when meconium in the amniotic fluid enters a baby’s lungs just before, or during delivery. Meconium is a baby’s first poo and is usually not present in the amniotic fluid. Most of the time, a baby passes meconium for the first time after delivery, but in some situations, babies can pass meconium before delivery whilst still inside the womb.

Aspiration (inhalation) of meconium can result in respiratory complications that could become life-threatening to babies if left unchecked. It is crucial to recognise the indicators of MAS as soon as possible, allowing healthcare professionals to provide the necessary support to parents and babies in order to prevent severe complications.

This article will explore the potential causes and risk factors of Meconium Aspiration Syndrome (MAS), the signs and symptoms to look out for, and the key aspects of its management.  

What is meconium?

Meconium is the first poo that a baby will pass,1 it begins to build up in a foetus’ gut between 12 and 16 weeks gestation and can even be visible via an ultrasound scan.2 Ideally, the baby should pass meconium between 24 and 48 hours after birth.

In reality, the passage of meconium is reassuring for healthcare professionals and parents alike - it indicates that the baby’s intestines and anus are correctly formed and are functioning well. Although, the passage of meconium does not exclude all bowel and gut problems. For example, if a newborn has not passed meconium within 48 hours after birth, this could indicate that the anus is not patent (open), meaning that the baby is not able to poo at all, and this can cause serious problems. 

What does meconium look like?

It is a dark, thick, and sticky substance, and often has a green tinge due to its high bile content. It also contains materials such as water, dead skin cells, hair, and other waste material.3 A poo that looks this way is very different to normal adult poo and can be alarming if you are unfamiliar with meconium. 

How does meconium aspiration happen?

As mentioned, babies can pass meconium whilst they are still in the womb. This results in meconium-stained amniotic fluid (MSAF). This usually occurs when the baby is under increased stress during delivery. Foetal distress can be caused by placental insufficiency, umbilical cord compression, pre-eclampsia, and maternal substance abuse.4

During pregnancy 

A baby doesn’t need to breathe as they receive all the oxygen they need from their mother’s blood via the placenta. Their waste carbon dioxide is also transported into the mother’s blood. This is why babies don’t drown inside the womb, even though their lungs are filled with amniotic fluid.

During labour

If you notice dark lumps or green staining in the amniotic fluid (waters), mention this to a healthcare professional as soon as possible. 

Women presenting with meconium-stained amniotic fluid are placed on a labour ward under the care of an obstetrician (a consultant doctor specialising in antenatal care and labour), just in case the baby develops difficulties during delivery. The labour ward will allow for closer continuous monitoring of mother and baby, so any worrying signs will be picked up early.5

During delivery

As the baby moves from the womb to the outside world, they must begin to breathe air. To do so, they must push all the amniotic fluid from their lungs to be able to properly get oxygen into their bloodstream. When a baby is distressed inside the womb, they gasp, causing fluid to enter the lungs and airways again.6

However, if a baby has passed meconium inside the womb, the amniotic fluid can become thick, and it could be more difficult for the baby to transition to breathing air and get all the fluid out of their airways. Some meconium may even remain in the airways or the alveoli. This is when problems develop and meconium aspiration syndrome starts.4

How can meconium impair a baby’s respiratory function?

  1. Physical obstruction: meconium can form a plug which blocks the airways and prevents air from reaching or leaving the lungs. Sometimes a meconium plug can cause a lung to collapse7
  2. Inactivation of surfactant: meconium acts on the pulmonary surfactant which prevents the alveoli (air sacs) of the lung from collapsing. When alveoli collapse, they can no longer extract oxygen from the air and transport it into the bloodstream7,8
  3. Inflammation of the airways: meconium and its contents irritate the airways, causing them to become inflamed and less able to help oxygen from the air enter the bloodstream and remove carbon dioxide from the blood7

What makes meconium aspiration syndrome more likely to happen?

Meconium aspiration syndrome only occurs when meconium-stained amniotic fluid is present, so its presence increases the chances of a baby developing the syndrome. Some factors increase the chance of developing the condition after meconium staining, but only around 4% of babies develop the associated syndrome:4,6

Factors that are associated with an increased risk of MSAF:

  • Post-term birth (past 42 weeks’ gestation): MSAF was present in 44% of births after 42 weeks2,4,7
  • Term gestation at birth (37 to 42 weeks gestation)
  • Foetal hypoxia: can be caused by disruptions in the placental circulation, like pre-eclampsia, placental insufficiency, and placental abruption9
  • Chorioamnionitis: an infection of the membranes surrounding the foetus
  • Thick meconium staining: If the meconium passed in the womb is especially thick, this can be even harder for babies to push out of their airways, whilst less thick meconium is likely to pass out of the airways more easily. Thick meconium staining is also associated with severe MAS6
  • Heart rate abnormalities detected whilst in the womb
  • Instrument-assisted delivery
  • Emergency caesarean section
  • Low APGAR score at 5 minutes of life

However, the presence of meconium-stained amniotic fluid does not necessarily mean that a baby will go on to develop meconium aspiration syndrome. Not all babies born through meconium-stained amniotic fluid will become unwell. Even if they do develop the condition, some babies will only be mildly affected. 

Signs and symptoms of meconium aspiration syndrome

So, we’ve covered what meconium is, how it gets into the lungs, and the risk factors associated with MAS. Now, let’s explore the signs and symptoms a baby with meconium aspiration syndrome may show.

The condition will present like any respiratory distress syndrome, with babies showing:

  • Fast breathing
  • Chest recessions: they’re having to work harder to breathe
  • Grunting while breathing
  • Slow heart rate
  • Blue skin or lips, if not enough oxygen is getting to the tissues
  • Limp or weak movements

Meconium aspiration syndrome can present with varying severities. Milder presentations of the syndrome may only demonstrate fast breathing, while some may present with severe respiratory failure, persistent pulmonary hypertension of the newborn (PPHN), and air leak syndromes. Severe presentations are life-threatening and require intensive care to reduce the chance of death.

Diagnosis of meconium aspiration syndrome

Meconium aspiration syndrome is defined as respiratory distress in a baby born in meconium-stained amniotic fluid when there is no other feasible cause.7 This is why the condition can only be diagnosed if a baby experiences persistent respiratory distress and any other possible causes have been ruled out.

Diagnostic tools:

  • Chest X-rays: an X-ray is used to visualise the baby’s chest and airways
  • Blood tests: to analyse oxygen and carbon dioxide levels
  • Arterial blood gases (ABG): a type of blood test used to analyse blood in the arteries
  • Pulse oximetry: a small device uses harmless infrared light to measure the levels of oxygen in capillary blood
  • Echocardiogram: an ultrasound scan of the heart and its chambers
  • Blood cultures: a special type of blood test used to check for bacteria and infection in the blood

These tests can both indicate the severity of any respiratory distress, as well as rule in or rule out any other potential causes for the baby’s breathing difficulties. For example, an echocardiogram may indicate that a heart problem is the underlying cause for the baby’s respiratory distress.

Management of meconium aspiration syndrome

The severity of respiratory distress usually determines the management plan, as well as where the baby will be treated, and by whom.

If a baby is born with meconium-stained amniotic fluid

They will be monitored closely on the labour ward. A midwife or nurse will take frequent readings of the baby’s observations in order to check that they are not declining and developing the condition.

If a baby is born with meconium-stained amniotic fluid and shows signs of respiratory distress

If a baby shows signs of respiratory distress, their management and location of treatment is dependent on the severity of their symptoms. A baby’s care is transferred over to a neonatologist (a consultant doctor specialising in newborns). A baby presenting with mild fast breathing will likely be transferred to the neonatal unit for closer monitoring and supplementary oxygen.

When a baby demonstrates severe respiratory distress, they are likely to be admitted into the neonatal intensive care unit (NICU) for even closer, continuous monitoring and administration of treatments that aren’t normally available on typical postnatal wards. Intensive care sounds scary, but your baby will receive attentive care and closer monitoring during their time on NICU.

Management of meconium aspiration syndrome is mostly supportive, and focuses on maintaining oxygen saturation levels to prevent brain injury:

  • Oxygenation: Babies with any respiratory distress will usually require supplemental oxygen to maintain adequate oxygen levels
  • Mechanical ventilation: A machine is used to take over breathing for the baby, if breathing has become too difficult. This is reserved for more severe cases
  • Suction: If meconium has caused an airway obstruction, it can be suctioned out using a machine. This, however, is not recommended unless there is a clear obstruction due to the risk of further complications4

Additional treatments can include:

  • Inhaled nitric oxide: This gas can assist in the management of persistent pulmonary hypertension of the newborn (a severe complication of MAS)
  • Surfactant: Babies may also receive surfactant, which is inactivated in meconium aspiration syndrome, to keep their alveoli from collapsing. Administration of surfactant is not standard but has been shown to be potentially beneficial in the management of the condition7


Acute complications

When meconium obstructs the airways, pressure can build ahead of and behind the blockage, causing pressure to build within the lungs. The resulting conditions are called air leak syndromes, and includes pneumothorax, pneumomediastinum, and pulmonary interstitial emphysema.10 These conditions occur when the increased pressure within the lungs causes air to ‘leak’ out of the lungs into surrounding tissues.

Persistent pulmonary hypertension of the newborn (PPHN) occurs when persistently low oxygen levels cause the blood vessels supplying the lungs to constrict. This causes the pressure in the pulmonary vascular system to rise, and the blood supply to the vital organs is limited. This is a very serious, life-threatening complication, but it occurs rarely (0.2% of all live births).

Chronic complications

Long-term impacts associated with the condition are more closely related to the effects of severe, prolonged poor oxygen supply to the brain. Most often, the lungs recover well, and without any permanent damage, unless any additional underlying conditions are diagnosed.


How common is MAS?

  • Meconium-stained amniotic fluid is present in around 4-22% of normal pregnancies6
    • 4% of those babies develop MAS4

How can I prevent MAS?

  • You cannot prevent MAS, but there are precautions that can help to reduce the harm associated with the condition.
    • If you notice green staining or thick, dark lumps in your waters (amniotic fluid), raise this with your midwife or closest healthcare professional. This will alert the labour team to the potential risk of MAS developing following birth and they can monitor the baby more closely after birth

How long does it take to recover from MAS?

  • Recovery from MAS is variable and depends on the severity of the condition. Babies on NICU who had severe MAS, with many complications, will take longer to recover than a baby with MAS who only experienced mild fast breathing for a short period of time


Meconium aspiration syndrome (MAS) is a respiratory distress syndrome that presents with varying severities in babies who were born in meconium-stained amniotic fluid. Early recognition of the condition is key and can allow for prompt management and intervention to prevent life-threatening complications. If untreated, complications such as persistent pulmonary hypertension of the newborn and pneumothorax can arise. The syndrome is more common in term and postterm babies (babies born between 37 and 42 weeks and 42+ weeks, respectively).

If you notice meconium in the amniotic fluid (waters) during delivery, raise this with the nearest healthcare professional immediately.

If you notice that your baby is experiencing breathing difficulties of any kind after birth, especially if there was stained meconium, raise your concerns with the nearest healthcare professional immediately.


  1. Skelly CL, Zulfiqar H, Sankararaman S. Meconium. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Jul 28]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK542240/
  2. Righetti C, Peroni DG, Pietrobelli A, Zancanaro C. Proton nuclear magnetic resonance analysis of meconium composition in newborns. J Pediatr Gastroenterol Nutr. 2003 Apr;36(4):498–501.
  3. Gallo DM, Romero R, Bosco M, Gotsch F, Jaiman S, Jung E, et al. Meconium-stained amniotic fluid. American Journal of Obstetrics and Gynecology [Internet]. 2023 May [cited 2023 Aug 3];228(5):S1158–78. Available from: https://linkinghub.elsevier.com/retrieve/pii/S0002937822021718 
  4.   Olicker AL, Raffay TM, Ryan RM. Neonatal respiratory distress secondary to meconium aspiration syndrome. Children (Basel) [Internet]. 2021 Mar 23 [cited 2023 Aug 3];8(3):246. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8005197/ 
  5. National Institute for Health and Care Excellence. If there is meconium during labour | Information for the public | Intrapartum care for healthy women and babies | Guidance | NICE [Internet]. 2014 [cited 2023 Aug 3]. Available from: https://www.nice.org.uk/guidance/cg190/ifp/chapter/if-there-is-meconium-during-labour 
  6. Fischer C, Rybakowski C, Ferdynus C, Sagot P, Gouyon JB. A population-based study of meconium aspiration syndrome in neonates born between 37 and 43 weeks of gestation. Int J Pediatr. 2012;2012:321545.
  7. Sayad E, Silva-Carmona M. Meconium aspiration. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Aug 3]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK557425/ 
  8. Lopez-Rodriguez E, Echaide M, Cruz A, Taeusch HW, Perez-Gil J. Meconium impairs pulmonary surfactant by a combined action of cholesterol and bile acids. Biophysical Journal [Internet]. 2011 Feb [cited 2023 Aug 3];100(3):646–55. Available from: https://linkinghub.elsevier.com/retrieve/pii/S0006349510052598 
  9. Hutter D, Kingdom J, Jaeggi E. Causes and mechanisms of intrauterine hypoxia and its impact on the fetal cardiovascular system: a review. Int J Pediatr. 2010;2010:401323.
  10. Whitfield JM, Charsha DS, Chiruvolu A. Prevention of meconium aspiration syndrome: an update and the Baylor experience. Proc (Bayl Univ Med Cent). 2009 Apr;22(2):128–31.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Grace Olatunde

Student - Bachelor of Medicine, Bachelor of Surgery – MBChB (Medicine) degree, Aston Medical School

Grace is a medical student due to enter her final year of study. She has a passion for patient education and is especially interested in the field of Urology.

She has several years of clinical experience in both public and private sector practice.

She is currently undertaking a PGCert in Health Leadership which she is due to complete in Summer 2024.

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