Meconium is a thick, sticky, greenish-black substance. It is the medical term for the newborn infant's first stools. Meconium is made of amniotic fluid, mucus, lanugo (the fine hair that covers the baby's body), bile, and cells that have been shed from the skin and the intestinal tract. . Infant stools typically change from meconium to seedy/mustardy yellow/green stools in 4-5 days.

During pregnancy, the baby floats in the amniotic fluid that fills the mother's uterus. This fluid protects the baby while he or she grows and develops. The baby swallows the amniotic fluid, which contains all the other constituents mentioned above. All of the contents other than the amniotic fluid itself are filtered out and remain behind in the intestine while the amniotic fluid is absorbed and re-released into the uterine space when the fetus urinates. This cycle maintains the amniotic fluid in a clear, healthy state during the nine months of pregnancy. This process of recycling the amniotic fluid occurs about every 3 hours.

In some cases, the baby passes stools (meconium) while still inside the uterus, and it is possible for the baby to breathe the meconium into the lungs. For more information on this condition, see meconium aspiration.