Meconium staining of the amniotic fluid with possibility of aspiration occurs in approximately 5% to 10% of births. Aspiration typically occurs when the fetus is stressed during labor. The infant is often post term (more than 40 weeks gestation). The condition is serious: meconium aspiration is a leading cause of severe illness and death in the newborn.

During labor, the infant may suffer a lack of oxygen. This can cause increased movement of the intestines (peristalsis) and relaxation of the anal sphincter, resulting in passage of meconium into the amniotic fluid surrounding the unborn baby. The amniotic fluid and meconium mix to form a green stained fluid of various thickness (viscosity). Meconium itself is thick, sticky and greenish-black in color and may be seen in the amniotic fluid after 34 weeks gestation.

If the infant breathes while still in the uterus or while still covered by this fluid after birth, the meconium/amniotic fluid mixture can be inhaled into the lungs. The inhaled meconium can cause a partial or complete blockage of the airways, causing difficulty breathing and poor gas exchange in the lungs. In addition, the substance is irritating and causes inflammation in the airways and potentially, chemical pneumonia.

About one third of the infants with meconium aspiration will require some type of assisted breathing. Risk factors include maternal diabetes, maternal hypertension, difficult delivery, fetal distress and intra-uterine hypoxia (decreased oxygen to the infant while it is still in the uterus).