Meconium: what is a baby's first stool?

Meconium is the initial substance present in the intestines of the developing foetus and constitutes the newborn's first bowel movement. It can be green, brown or yellow. 
 

It first forms in the gastrointestinal tract of a foetus between 11 and 14 weeks of pregnancy [1]. 
 

Healthy full-term newborns pass meconium between 24 and 48 hours after birth. Premature infants generally show a delayed passage. The passage of meconium within 24–48 hours of birth indicates that the newborn's intestines are intact and patent [2]. 

The absence of meconium passage beyond 48 hours in full-term newborns may indicate a disease or obstruction of the infant's intestine [3].
 

In the event of rupture of the foetal membranes, the midwife or nurse must assess the colour of the amniotic fluid. The amniotic fluid should be clear, or straw-tinged with small particles of vernix in the liquid. A brown or green discolouration of the amniotic fluid indicates the passage of meconium in utero. 

Meconium can be released by the baby in utero, before birth. The presence of meconium in the amniotic fluid affects approximately 12 to 16% of deliveries [4]. Overall, it is observed in 5% of cases before 37 weeks of pregnancy, in 25% of term births, and in up to 52% of post-term pregnancies [1].

The passage of meconium is rare before 34 weeks and its incidence increases steadily beyond 37 weeks of pregnancy. It may simply represent normal gastrointestinal maturation or indicate a hypoxic event (lack of oxygen), making it a warning sign of foetal compromise [4].

Meconium aspiration syndrome occurs when meconium is found below the vocal cords. This occurs in approximately 10% of cases of meconium in utero and is associated with 20% neonatal mortality [1]. 
 

This is a rare complication — the presence of meconium in utero occurs on average in 14% of pregnancies, and only 2% of babies with meconium in utero are at risk of mortality associated with meconium aspiration syndrome!

Aspiration can occur in utero during foetal gasping, or after birth during the first breaths of life. Meconium aspiration syndrome is defined as respiratory distress developing shortly after birth, with radiographic evidence of aspiration pneumonia and the presence of meconium-stained amniotic fluid [9]. Amniotic fluid containing thick meconium is more likely to lead to respiratory symptoms [6].

Meconium refers to a baby's first stools. Healthy full-term newborns pass meconium within 24 to 48 hours of birth, indicating that the newborn's intestines are intact and patent.
 

The release of meconium in utero, before labour, may simply represent normal gastrointestinal maturation in your baby, but it can also be a sign of hypoxia. This can, for example, lead to respiratory distress.