Meconium is the medical term for a newborn’s first stool, which is a dark green, viscous substance. When a baby passes this stool while still in the uterus, it mixes with the amniotic fluid. Meconium aspiration syndrome (MAS) occurs if a newborn breathes this mixture into their lungs shortly before, during, or after birth, leading to respiratory problems. While the presence of meconium-stained amniotic fluid can be a sign of fetal distress, it does not automatically mean the baby will develop MAS.
Mortality Rate of Meconium Aspiration
The mortality rate for meconium aspiration syndrome has seen a significant decline over the past few decades due to advances in obstetrical and neonatal care. In developed countries, the overall death rate directly attributable to MAS is now approximately 1.2% to 2.5%, a sharp reduction from rates as high as 40% reported in the 1970s. Meconium-stained amniotic fluid (MSAF) occurs in about 8-25% of all births after 34 weeks of gestation.
Of those infants born through meconium-stained fluid, only a small fraction, around 5%, will develop meconium aspiration syndrome. The reduction in mortality is largely credited to changes in medical practice, such as a decrease in pregnancies continuing beyond 41 weeks. However, in cases where MAS is severe and results in lung tissue disease and persistent pulmonary hypertension, the death rate can increase to as high as 20%.
Factors Influencing Severity and Risk
A primary determinant of severity is the consistency of the meconium; thick, dense meconium is more likely to cause significant airway obstruction compared to thin, watery meconium. This obstruction can lead to a “ball-valve” effect, where air is trapped in the lungs, or complete blockage, causing lung collapse.
A primary complication is Persistent Pulmonary Hypertension of the Newborn (PPHN), which occurs when the infant’s circulatory system fails to adapt to breathing outside the womb. This causes high blood pressure in the lung’s arteries, severely limiting the baby’s ability to oxygenate their blood. Meconium also triggers an inflammatory response in the lung tissue (chemical pneumonitis), which can inactivate the surfactant needed to keep the small air sacs in the lungs open.
Medical Interventions and Management
The management of an infant with MAS begins immediately after birth. If the baby is born through meconium-stained fluid but is active and crying, no invasive procedures may be needed. However, if the infant shows signs of respiratory distress, the first step is to suction the mouth and nose. In some cases, a tube may be inserted into the windpipe to remove deeper secretions.
Infants with more than mild symptoms are admitted to a neonatal intensive care unit (NICU) for close monitoring. Many infants only require supplemental oxygen, while others may need continuous positive airway pressure (CPAP) to help keep the lungs expanded. For about a third of infants with MAS, mechanical ventilation is necessary to support breathing.
Advanced therapies include surfactant replacement, which can be administered directly into the lungs to counteract the deactivating effects of meconium. If PPHN develops, inhaled nitric oxide is used to relax the blood vessels in the lungs and improve blood flow. In situations where these interventions are not enough, Extracorporeal Membrane Oxygenation (ECMO) can be used to temporarily do the work of the heart and lungs.
Long-Term Outlook for Survivors
The vast majority of infants with mild to moderate MAS recover completely with no lasting health problems. Breathing difficulties resolve within a few days to weeks, and most do not suffer permanent lung damage.
For infants who survive severe MAS, there is an increased risk of future respiratory issues. These children may have a higher incidence of reactive airway disease, experience wheezing during early childhood, or have more sensitive lungs that could lead to a condition similar to asthma.
In the most severe cases involving prolonged lack of oxygen, there is a risk of developmental abnormalities or brain damage. These outcomes are not common and are associated with the underlying fetal distress that led to the meconium passage, rather than the aspiration itself. These children are monitored by pediatric specialists to manage potential long-term effects.