Gas can begin the first day of a newborn’s life, as their digestive systems are just developing, and they do not have strong systems to deal with reflux and swallowed air. Gas can occur in a baby for several reasons, but mostly it is because they have swallowed excess air when crying or feeding. Their delicate little systems do not know how to get rid of this gas, and the pain from it makes them cry and become fussy. They will often pull up their legs to their stomachs to help expel the gas on their own. The critical time for gas in infants is generally from newborn to four-months-old but can last longer than that depending on the development of the digestive system.