Meconium is the first stool of an infant, composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, aminotic fluid, bile, and water. Meconium is sterile, unlike later feces, and has no odor.

Meconium is normally stored in the infant's intestines until after birth, but sometimes it is expelled into the amniotic fluid prior to birth, or during labor. If the baby then inhales the contaminated fluid, respiratory problems may occur, which are known as meconium aspiration syndrome (or MAS).