Meconium is the baby's first feces, or poop, which is sticky, thick, and dark green and the most obvious sign that meconium has been passed during or before labor is the greenish or yellowish appearance of the amniotic fluid. When the thick meconium mixes into the amniotic fluid, it is swallowed and breathed into the airway of the fetus.

This is known as meconium aspiration or meconium aspiration syndrome (MAS). Although MAS is often not life-threatening, it can cause significant health complications for your newborn. And, if MAS is severe or untreated, it can be fatal.