Meconium is the earliest stool of an  infant. Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water. Meconium, unlike later feces, is viscous and sticky like tar, its color usually being a very dark olive green; it is almost odorless. When diluted in amniotic fluid, it may appear in various shades of green, brown, or yellow. It should be completely passed by the end of the first few days after birth, with the stools progressing toward yellow (digested milk).

Meconium is normally retained in the infant’s bowel until after birth, but sometimes it is expelled into the amniotic fluid prior to birth or during labor and delivery. The stained amniotic fluid  is recognized by medical staff that this may be a sign of fetal distress. Some post-dates pregnancies (where the woman is more than 40 weeks pregnant) may also have meconium stained liquor without fetal distress. Medical staff may aspirate the meconium from the nose and mouth of a newborn immediately after delivery in the event the baby shows signs of respiratory distress to decrease the risk of meconium aspiration syndrome.