Defining mattersThe first English-language pediatrics textbook, published in 1544, labels colic as "a rumbling in the gut," and even now doctors think there's something to that definition. Colic tends to strike before a baby's nervous and digestive systems are fully developed, says Marc Weissbluth, M.D., associate professor of pediatrics at Northwestern Medical School, in Chicago. So perhaps it's not surprising that during this period, some infants have more powerful intestinal contractions than others and act like their tummies hurt. Their legs stiffen, their bellies swell, and they tend to pass a lot of gas when they cry.
According to Dr. Weissbluth, some tests show that babies with colic have elevated levels of serotonin, a brain chemical that can trigger these intestinal muscle spasms. The colic cry is also very similar to a cry caused by pain, notes Barry Lester, Ph.D., director of the Infant Development Center at the Women and Infants Hospital at Brown University School of Medicine. "Both cries are sudden, loud, high-pitched, and involve lots of breath holding."
Another new theory with researchers: Colicky infants are just poor sleepers. "These babies are more sensitive to noise, light, and other stimuli than other infants," says Maureen Keefe, Ph.D., dean of the College of Nursing at the Medical University of South Carolina, in Charleston. "Our research indicates that they spend more time in 'active' sleep, than in 'deep' sleep. So they tend to have a tougher time falling and staying asleep." There's also some evidence that babies who suffer from colic may have low levels of melatonin, a hormone that helps regulate sleep.