If a newborn baby has mucus in the mouth, it is mainly considered due to the following reasons: first, it is caused by swallowing amniotic fluid. If a newborn swallows amniotic fluid while in the mother’s womb or just after birth, there will be mucus in the mouth, which is a normal phenomenon. If the child is in good spirits, most of it will disappear slowly and parents need not worry too much. Secondly, it is caused by respiratory tract infection. If the child has mucus in the mouth, accompanied by a cough and fever, and the number of breaths increases, it is often due to bronchitis or bronchopneumonia. Finally, it is due to gastroesophageal reflux. If your child has GERD, he or she will also have mucus in the mouth and often spit up.