Colic often starts a few weeks after birth, peaking at about 5 to 8 weeks of age. It usually goes away by 4 months of age. Babies' symptoms include irritability, inconsolable crying, red face, clenched fists, drawn-up legs, and screaming.
Colic affects an estimated 5%-28% of babies born in Western countries. Its causes have been attributed to everything from exposure to cow's milk proteins to feeding difficulties to maternal depression or anxiety.
Not all of those theories have been scientifically confirmed. Shenassa and Brown say colic probably has multiple independent causes.
The review was conducted by Edmond Shenassa, ScD, of the community health department at Brown Medical School, and Mary-Jean Brown, ScD, RN, of the society, human development, and health department of Harvard School of Public Health. Their report appears in the October issue of the journal Pediatrics.
Smoking has also been linked to colic; Shenassa and Brown's review supports that idea. "Exposure to cigarette smoke and its metabolites may be linked to infantile colic," they write.
Tobacco smoke appears to raise levels of a gut hormone called motilin in the blood and intestines. Motilin increases the contractions of the stomach and intestines, increasing the movement of food through the gut.
"Higher-than-average motilin levels are linked to elevated risks of infantile colic," say the researchers, who can't yet trace the exact chain reaction that links motilin to colic.
They also don't know when tobacco smoke starts raising the risk of colic.
Babies can be exposed to smoke while they're still in the womb, through breast milk, or by being around a smoker after birth.
Nearly half of all U.S. women who smoke keep on smoking through their pregnancies, according to the researchers. That's nearly 12% of all women who give birth.
"More than 500,000 infants each year are exposed to cigarette smoke in utero," say Shenassa and Brown.
Some research indicates that colic's impact can linger.
Once-colicky babies may also have more feeding difficulties, and the stress of tending to a colicky child could affect a caregiver's relationship with the baby. That's all the more reason to quit smoking and avoid tobacco smoke, say the researchers.
"Decreased exposure to tobacco smoke can be expected to provide widespread, long-term health benefits to maternal and child populations," they conclude.