Babies exposed to their mother's cigarette smoke in the womb later perform more poorly on reading comprehension tests, according to a new study.
"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," lead author Dr. Jeffrey Gruen of Yale University told Reuters Health.
In the study, researchers found that children born to mothers who smoked more than one pack per day struggled on tests specifically designed to measure how accurately a child reads aloud and if she understands what she read.
On average, children exposed to high levels of nicotine in utero -- defined as the minimum amount in one pack of cigarettes per day -- scored 21 percent lower in these areas than classmates born to non-smoking mothers. The difference remained even when researchers took other factors -- such as if parents read books to their children, worked in lower-paying jobs or were married -- into account.
Put another way, among students who share similar backgrounds and education, a child of a smoking mother will on average be ranked seven places lower in a class of 31 in reading accuracy and comprehension ability, said co-author Jan Frijters of Brock University in Ontario, Canada.
Previous studies have found smoking during pregnancy is linked to lower IQ scores and academic achievement, and more behavioral disorders. The authors found no reports so far that zeroed in on specific reading tasks like accuracy and comprehension in a large population.
The team, which published their results in The Journal of Pediatrics, pulled data from more than 5,000 children involved in the Avon Longitudinal Study of Parents and Children (ALSPC) study that began in the early 1990s in the UK. Only data from children with IQ scores of 76 and higher were used. An IQ score of 70 and below can be the sign of a mental disability.
UK researchers collected questionnaires from mothers before and after giving birth. This helps make the self-reported data more trustworthy, explained Sam Oh of the University of California, San Francisco, who wasn't involved with the work. If mothers knew their child's reading scores beforehand, they might subconsciously report more or less smoking.
"To me, this study suggests that the effects attributed to in utero smoking can in fact be attributed to the intrauterine environment, and not due to environmental differences that the children grow up in," Oh told Reuters Health by email.
Large observational studies like this one call attention to patterns, but do not prove a direct cause-effect relationship between cigarette smoking and low reading scores.
Despite public health initiatives to discourage smoking, as many as one in six pregnant American women still light up, according to national surveys by the Centers for Disease Control and Prevention and the U.S. Department of Health and Human Services.
"That is a lot of children," Dr. Tomáš Paus of the University of Toronto told Reuters Health.
Paus added that the study tied the effects of low test scores to nicotine in cigarettes, which also produce other harmful chemicals and carbon dioxide. Either way, smoking while pregnant seems to put a baby at risk for negative health outcomes.
"We should not be happy with those rates. Smoking during pregnancy is preventable," Paus said.