Immediate Cause

The immediate cause is the release of meconium by a fetus while in the uterus or the birthing process and the baby’s breathing in of the meconium due to an instinctive gasping.

Meconium is a greenish fluid that fetuses can produce after their 35th week in the womb, as a fetus’s digestive functions have matured enough to produce waste. Akin to fetal matter, meconium is different to stool because it is sterile. Meconium aspiration also typically only happens after 35 weeks because, by that point, the baby has developed certain primary instincts, such as gasping for breath, which they can do in response to a shortage of oxygen.

Meconium is typically excreted after birth. However, in around 20% of cases, if a fetus experiences stress or distress (typically if blood or oxygen supply drops), the baby can pass meconium while in the womb. The meconium then mixes with the surrounding amniotic fluid. A fetus could then aspirate the meconium if it has matured enough and developed a reflexive gasping motion. This gasping reflex allows the meconium to enter into the lungs along with the amniotic fluid (whereas typically, fluids do not pass the trachea before birth). Alternatively, if a fetus had expelled meconium in the womb, during and immediately after birth, it could be covered with the amniotic fluid that was mixed with meconium. The newborn baby could then inhale some meconium along with its first few breaths upon birth. Approximately 10% of babies who expel meconium as a fetus will aspirate it (before or after delivery).

Causes of Gestational Distress

As noted above this can occur when a fetus experiences distress in the womb, which triggers an early release of meconium. Usually, a shortage of oxygen is both what causes the stress that triggers the early meconium release as well as makes the gasping reflex kick in, by which the meconium enters the fetus’s breathing passage and the lungs. There are a few reasons for such an oxygen shortage.

A few reasons for an oxygen shortage that can trigger meconium aspiration is issues in the oxygen delivery system to the baby. For example, if a fetus’s delivery date has come and gone, meconium release and aspiration could be triggered by an aging placenta. After 40 weeks, the placenta loses some of its efficiency in delivering oxygen to the fetus. Moreover, as the fetus is in the womb for longer periods past the expected due date, the less placenta surrounds the fetus, leading to an increased concentration of meconium that is in the amniotic fluid. Along with the fetus’s enhanced internal functions (digestive functions and gasping reflex), the decreased efficacy of the aging placenta, as well as the decrease in amount of placenta, can make meconium aspiration more likely.

Similarly, the umbilical cord is the fetus’s lifeline and any issues or compression of the umbilical cord could not only hinder the delivery of vital nutrients to the fetus but also the delivery of oxygen. Any interruptions or issues with oxygen delivery via the umbilical cord (for example, if the umbilical cord is pinned between the uterine wall or the pelvis and the baby) can also promote gestational distress that may trigger meconium release and aspiration.

A fetus that has an underdeveloped or impaired ability to process oxygen (i.e. because the circulatory system has not developed properly or the baby has high blood pressure) can also be more likely to experience distress, and thus be at a higher risk of meconium aspiration.

Similarly, if the mother has health issues that affect blood circulation, these issues could also impede oxygen delivery to the baby – causing meconium release and aspiration. For example, mothers who have diabetes, have chronic respiratory disease, cardiovascular disease, or high blood pressure may be more likely to have babies with MAS, because these health issues may make it more difficult for the fetus to get the oxygen it needs. Similarly, a fetus may be more likely to experience oxygen shortage if the mother is a heavy smoker or uses drugs (particularly cocaine). An infection in the mother or the fetus could also make it more difficult for the baby to deliver or receive oxygen and thus trigger the gasping reflex.

Finally, a long or difficult delivery may result in more stress on the fetus, causing excretion of the meconium and trigger the gasping reflex and breathing in of any meconium that was already released and mixed into the amniotic fluid.