CHICAGO, Illinois (AP) -- Mothers who smoke during or after pregnancy increase their babies' risk of developing colic, those vexing, inconsolable crying spells that affect up to 20 percent of U.S. babies in their first few months of life, researchers say.

The culprit, based on studies in adults, is likely nicotine, which increases blood levels of a gut protein involved in digestion, said Brown University epidemiologist Edmond Shenassa. That may result in painful cramping that makes babies cry, he said.

Shenassa and Harvard University researcher Mary-Jean Brown reviewed several studies, including six that involved more than 12,000 babies.

The data suggest that compared with nonsmokers, mothers who smoke during pregnancy face about double the risk of having infants with colic, Shenassa said.

Secondhand smoke -- from parents and others who light up around the baby -- also appears to increase the risk for colic, but Shenassa said more research is needed to sort out how much those factors increase the risk.

Smoking by mothers already has been linked with an increased risk for low birth weight, sudden infant death syndrome and respiratory problems in infants.

"If, as we suspect, exposure to cigarette smoke increases the risk of colic, then this would provide additional incentives to parents to abstain from smoking," the researchers said.

The report appears in the October edition of Pediatrics, published Monday.

Classic colic -- crying spells occurring at least three hours daily, at least three times weekly, for at least three weeks -- tends to peak at two months and gradually disappear by about three or four months of age.

Other research has suggested that some cases may be caused by an inability to properly digest milk proteins or fruit-juice sugars, though some scientists believe colic is normal behavior for some babies that may be exacerbated but not caused by outside influences.

Shenassa said evidence of nicotine increasing levels of the protein motilin, which is involved in controlling intestinal activity, comes from studies of adult smokers.

Dr. Ronald Barr, a University of British Columbia pediatrics professor who was not involved in the research, called the paper "a very nice review of the literature" and said it provides sound reasons "to suggest that smoking might be contributory."

But Barr noted that some of the reviewed data showed that a sizable number of babies born to nonsmokers had colic, and he argued that smoking would not cause colic in infants who aren't already predisposed.

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