Congestion from a stuffy nose, a respiratory virus, bronchiolitis, postnasal drip from colds or throat infections, and influenza, a common ailment in newborns, can cause phlegm in a baby's throat, according to the Children's Hospital of the King's Daughters. Babies also develop phlegm when struggling from seasonal allergies, notes WebMD.
Phlegm in babies resulting from post nasal drip is often due to sinus infections, inflammation of the sinuses, objects stuck in the nose, and certain foods that trigger mucus flow, explains WebMD. Babies may also develop phlegm when exposed to irritants such as smoke, perfume, chemicals or cleaning products. Changes in weather, such as excess dryness or cold temperatures, may also cause babies to develop phlegm.