Meconium is a thick and sticky greenish black substance that forms in the intestines of a fetus. It is produced as the fetus swallows and digests the amniotic fluid inside the womb. When the baby is born, meconium is the first stool the baby passes.

Most normal newborns pass the meconium within 48 hours after birth. Some babies cannot pass the meconium because of a blockage in the intestines, known as a meconium plug. In babies born with cystic fibrosis, the intestines may become blocked from excessive mucus production. Stuck meconium must be flushed out by an enema. Sometimes the blockage is so severe that surgery is needed.