Diaper peeing and pooping. Elsevier Health Sciences. A Word From Verywell. Signs and symptoms usually begin with sudden vomitting, followed by severe diarrhoea. Day 2: It is from the hormones in your baby's body, and it's not harmful. If your baby's poo is normally brown or yellowish brown but turns abnormally black, seek medical help. These symptoms may indicate an intestinal condition that warrants attention from your doctor. Explore Merries Product Lineup. Many new moms have questions about their baby's urination pattern.