Meconium is a thick, greenish black substance that forms in the intestines of an unborn child (fetus). As the fetus swallows and digests the fluid (amniotic fluid) inside the womb, it produces meconium.

When the baby is born, meconium is the first stool the baby passes.

Meconium in the amniotic fluid may be a sign that the newborn experienced stress before birth. If meconium gets into the baby's lungs, it can cause irritation and inflammation of the lungs.