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Smoking is harmful to anyone. But the ill effects of maternal smoking are particularly severe, as the consequences and harm to the unborn child may last for a lifetime. A new research by scientists from Yale School of Medicine reveals that children born to mothers who smoked more than one pack per day during pregnancy have trouble in reading and comprehension.

A test conducted by the researchers showed that such children did not score well on tests designed to measure how accurately a child reads aloud and comprehends what they read, Yale News reported.

For the study, researchers analyzed data from more than 5,000 children. They compared the skills of seven specific tasks done by the children - reading speed, single-word identification, spelling, accuracy, real and non-word reading, and reading comprehension - with maternal cigarette smoking. During the study and while drawing conclusions, the researchers considered factors such as socioeconomic status, mother-child interactions and 14 other potential factors.

The findings of the study revealed that on an average, children who are exposed to high levels of nicotine in their mother's womb scored 21 percent lower in the reading and comprehension tests when compared to their classmates who were born to mothers who did not smoke during pregnancy. This amount of nicotine was defined as the amount in one pack of cigarettes per day. The tests were conducted when children were 7-year-olds, and again at age 9.

When other factors are constant, apparently, a child whose mother smoked during pregnancy will be, on an average, ranked seven places lower in a class of 31 in reading accuracy and comprehension ability, the report said.

"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," said lead author Dr. Jeffrey Gruen, professor of pediatrics and genetics at Yale School of Medicine.

Gruen also noted that the effects of maternal smoking are especially loud in children with an underlying phonological deficit. This suggests the link between smoking and phonological ability.

"The interaction between nicotine exposure and phonology suggests a significant gene-by-environment interaction, making children with an underlying phonological deficit particularly vulnerable," he said.

The study, published in the current issue of The Journal of Pediatrics, was co-authored by Kelly Cho, Jan C. Frijters, Heping Zhang, and Laura L. Miller.