Amniotic fluid plays a crucial role in fetal development as it contains not only the urine produced by the fetus but also occasionally the first feces known as meconium. This meconium is made up of various substances, including mucus, bile, epithelial cells, fetal hair, and remnants from the amniotic fluid. The composition reflects the conditions within the womb and the baby’s early digestive process.

The description in the book "Gulp: Adventures on the Alimentary Canal" by Mary Roach highlights the contrast between the appearance of meconium and the excrement of a breast-fed newborn. Meconium is typically a tarry, olive-brown color, while the waste from a breast-fed infant is yellowish. This comparison emphasizes the differences in infant digestion and excretion in their early life stages.