Colic is a problem that affects some babies during the first three to four months of life. It can be very stressful and frustrating to parents. Physicians have defined colic as prolonged or excessive crying in an infant who is otherwise well. The crying can be very loud and can last for several hours a day. Colic often starts by 3 weeks of age, is at its worst around 6 weeks, and gradually gets better by about 3 months of age. It is not clear what causes colic.