Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water.

Meconium, unlike later feces, is viscous and sticky like tar – its color usually being a very dark olive green and it is almost odorless.

This indicates that the fetus passed the meconium some time ago such that sufficient mixing occurred as to establish the homogeneous mixture.

[2] In cystic fibrosis, the meconium can form a bituminous black-green mechanical obstruction in a segment of the ileum.

Other researchers have raised questions about whether these findings may be due to contamination after sample collection and that meconium is, in fact, sterile until after birth.

[10] Further researchers have hypothesized that there may be bacteria in the womb, but these are a normal part of pregnancy and could have an important role in shaping the developing immune system and are not harmful to the baby.

[11] The Latin term meconium derives from Greek μηκώνιον, mēkōnion, a diminutive of μήκων, mēkōn 'poppy', in reference either to its tar-like appearance that may resemble some raw opium preparations or to Aristotle's belief that it induces sleep in the fetus.