Risk Factors for Meconium Aspiration Syndrome
The primary risk factors for meconium aspiration syndrome (MAS) include meconium-stained amniotic fluid in non-vigorous infants, post-term pregnancy, and fetal distress. 1
Understanding Meconium Aspiration Syndrome
Meconium aspiration syndrome occurs when a newborn inhales meconium-stained amniotic fluid into the lungs, causing respiratory distress. Meconium-stained amniotic fluid (MSAF) is present in approximately 5-15% of all deliveries, with about 3-5% of infants born through MSAF developing MAS 2.
Major Risk Factors
Maternal Factors:
- Post-term pregnancy (≥42 weeks gestation) 2, 3
- Maternal hypertension 3
- Maternal anemia 3
- Maternal smoking 3
- Certain ethnic backgrounds (higher risk noted in Maori population) 3
Fetal/Intrapartum Factors:
- Fetal distress/hypoxia 2
- Non-vigorous infant at birth (heart rate <100/min, decreased muscle tone, and/or depressed breathing) 1
- Thick meconium consistency 3
- Prolonged labor 3
- Late fetal heart decelerations 3
Physiological Factors:
- Maturation of gastrointestinal tract - meconium passage is rare in preterm infants but common near or post-term 2
- Reduced clearance of meconium due to impaired fetal swallowing 2
- Placental dysfunction 2
Clinical Implications
The American Heart Association guidelines highlight that depressed infants born through meconium-stained amniotic fluid are at increased risk of developing MAS 1. The presence of meconium in the respiratory tract leads to:
- Atelectasis
- Hypoxemia and hypercapnia
- Persistent pulmonary hypertension
- Inflammatory changes
- Surfactant inactivation 4
Management Considerations
Current guidelines recommend:
For vigorous infants with MSAF: Routine intrapartum oropharyngeal and nasopharyngeal suctioning is no longer recommended 1.
For non-vigorous infants with MSAF: The 2020 International Consensus suggests against routine immediate direct laryngoscopy with tracheal suctioning 1. This represents a change from earlier practices.
Team preparation: Because meconium-stained amniotic fluid increases the risk of requiring resuscitation, a team skilled in tracheal intubation should be present at birth 1.
Complications and Outcomes
MAS has a case fatality rate of approximately 5% (up to 40% in severe cases) 2. Long-term complications may include:
- Pulmonary sequelae
- Neurodevelopmental issues
- Higher prevalence of asthmatic symptoms and bronchiolar hyperreactivity later in life 2
Prevention Strategies
- Preventing prolonged pregnancy through appropriate labor induction may reduce the risk of MSAF 2
- Careful monitoring during labor with continuous fetal heart rate monitoring 4
- Examination of acid-base equilibrium in capillary blood from the fetal scalp when indicated 4
Understanding these risk factors is crucial for identifying infants at high risk for MAS and ensuring appropriate preparation and management at delivery to minimize morbidity and mortality.