Causes of Meconium Aspiration Syndrome
Before a baby is born, fluid usually moves in and out of the trachea (the upper part of the airway) only. Meconium can be inhaled into the lungs if the baby gasps while still in the womb or during the initial gasping breaths after delivery. This gasping usually happens because a problem (such as an infection or compression of the umbilical cord) made it hard for the baby to get enough oxygen before birth.

MAS is often related to fetal stress. This can be caused by problems in the womb, such as infections, or by difficulties during the birth. A distressed baby may have hypoxia (decreased oxygen), which can make the baby's intestinal activity increase and cause relaxation of the anal sphincter (the muscular valve that controls the passage of feces out of the anus). This relaxation then moves meconium into the amniotic fluid that envelops the baby.

But meconium passage during labor and delivery isn't always associated with fetal distress. Sometimes, babies who aren't distressed during labor pass meconium before birth. In either case, a baby that gasps or inhales meconium can develop MAS.

Other risk factors for MAS include:

a long or difficult delivery
advanced gestational age (or postmaturity)
a mother who smokes cigarettes heavily or who has diabetes, high blood pressure (hypertension), or chronic respiratory or cardiovascular disease
umbilical cord complications
poor intrauterine growth (poor growth of the baby while in the uterus)