Colic affects 20% of babies and is defined by its symptoms rather than a particular cause. A baby is described as having colic if they cry over 3 hours per day at least 3 days a week. Colic is usually found in babies age 2 weeks to 4 months. Most colicky babies cry more in the late evening or at night. Usually by the 4th month the infant stops crying, giving parents some much needed relief.

The actual cause of colic is unknown, but factors which may contribute to colic are intestinal gas pains, overfeeding, or nervous tension. Digestive dysfunction is supported by the fact that colicky babies typically pull their legs up as if experiencing abdominal cramps or pain. However, some experts suggest that abdominal pain is the result of crying rather than the cause as crying may result in swallowing air, with resultant stomach distention.

Another theory is that colicky babies are intolerant of certain types of food such as dairy. This may be the case if a baby cries more after feedings. Some researchers now suggest that some babies suffer from reflux, a condition in which stomach acid is regurgitated causing heartburn.