There is no agreed-upon definition for colic in babies. Most healthcare providers consider a baby to have colic when he or she cries for more than three hours straight at least three days per week for more than three weeks but has a clean bill of health otherwise. This is known as the "rule of three," or the Wessel criteria.
No one knows for sure what causes colic. Researchers have studied possible biological, social, psychological, and physical factors. But no single factor has been shown to cause colic. Babies with the condition are otherwise healthy, so it is not caused by a medical condition. Colic is also not caused by something that the parents are or are not doing.
You can take comfort in knowing that your baby will not have colic will not last forever. Typically speaking, the condition peaks around six to eight weeks and then goes away around the baby's third month. It often stops as mysteriously as it began.
(Click Infant Colic for more information on how healthcare providers make a diagnosis, tips for how to cope with a baby who has colic, and possible causes.)