Meconium

Meconium is the earliest stool of a mammalian infant, formed in utero from materials ingested during fetal life. In healthy pregnancies, it is typically passed in the first day or two after birth, usually within 24 to 48 hours. Passage before birth may occur in cases of fetal distress and is associated with meconium-stained amniotic fluid, which can pose a risk of inhalation by the newborn during delivery.

Meconium is thick, sticky, and varies in color from greenish-black to dark brown. It is generally odorless or faintly earthy. Its composition includes desquamated intestinal epithelial cells, mucus, swallowed amniotic fluid, bile pigments such as bilirubin and biliverdin with the final product stercobilin, and other secretions from the fetal gut. It is formed before birth and is typically sterile at the time of excretion.

Meconium formation occurs in the fetal intestine; colonization by bacteria occurs after birth. After birth and the onset of respiration and feeding, peristaltic activity increases and the meconium is gradually replaced by transitional stool and then by ordinary feces.

Clinical significance: Delayed passage beyond 48 hours may indicate underlying disorders such as Hirschsprung disease, hypothyroidism, or intestinal motility problems. Meconium ileus, a distal intestinal obstruction caused by thick, viscous meconium, is classically associated with cystic fibrosis. Meconium-stained amniotic fluid is linked with fetal hypoxia and can lead to meconium aspiration syndrome if inhaled, causing respiratory distress in the newborn.