TUESDAY, Nov. 27 (HealthDay News) -- Children whose mothers smoked one or more packs of cigarettes a day during pregnancy have poorer reading skills than other children, a new study finds.

Researchers analyzed data from more than 5,000 children in the United Kingdom, and compared their scores on a series of tests assessing how accurately a child reads aloud and comprehends what he or she reads. The children were tested at ages 7 and 9.

On average, children whose mothers smoked at least one pack of cigarettes a day during pregnancy scored 21 percent lower on the tests than children born to nonsmoking mothers, according to the study published this month in the Journal of Pediatrics.

In a class of 31 students with similar backgrounds and education, a child of a mother who smokes will rank an average of seven places lower in reading accuracy and comprehension ability than a child of a nonsmoking mother, the researchers concluded.

"It's not a little difference -- it's a big difference in accuracy and comprehension at a critical time when children are being assessed, and are getting a sense of what it means to be successful," study author Dr. Jeffrey Gruen, a professor of pediatrics and genetics at Yale School of Medicine, said in a Yale news release.

The effects of smoking during pregnancy are especially strong in children with an underlying speech disorder, which suggests an interaction between nicotine exposure in the womb and a highly heritable trait such as speech ability, Gruen added.

The study found an association between maternal smoking and children's reading skills, but it didn't prove cause-and-effect.

More information

The U.S. Centers for Disease Control and Prevention has more about the smoking during pregnancy.