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.

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.

When the baby is born, meconium is the first stool the baby passes. Most newborns pass the meconium within 48 hours after birth.

If your baby doesn't pass his or her first stool within the usual time frame, it may be a sign of a blocked intestine or another problem. Your baby may need treatment to remove the stool from the body.