Babies first stool. It begins to form in the gut between 12 and 16 weeks of pregnancy and is made up of what baby has ingested in utero, or what they have produced. This can include but is not limited to: amniotic fluid (definition link), lanugo (definition link), cells, water, and bile. It is very dark in colour and it is also very sticky. Your baby will be expected to pass meconium within the first 24 hours of life. In addition, up to 25% of babies beyond 34 weeks pass meconium whilst still in utero.