New York - Children of women who smoked cigarettes during pregnancy are more likely to have behavioural problems than those whose mothers didn't light up, says a new analysis.
"The evidence is emerging that smoking in pregnancy and the frequency of smoking in pregnancy is correlated with developmental outcomes after (children) are born," said Gordon Harold, the study's senior author from the University of Leicester in the UK.
Previous research has tied smoking cigarettes during pregnancy to behaviour problems among children later on, but those studies couldn't rule out the influence of other factors, such as genetics or parenting techniques, researchers said.
For example, Harold and his colleagues write in JAMA Psychiatry that mothers who smoke during pregnancy are more likely to have parenting styles that condone acting out.
For the new analysis, the researchers pulled together data from three studies from New Zealand, the UK and the US that asked women whether or not they smoked cigarettes during pregnancy. Parents and teachers then reported on children's conduct problems - such as getting in fights or having difficulty paying attention - between age four and 10.
The analysis also compared children who were raised by adoptive mothers to children who were raised by their biological mothers in an effort to tease out the influence of genetics and parenting styles on any link between prenatal smoking and behaviour.
The researchers compared children's behaviour scores to an average of 100, where higher scores indicate more conduct problems.
In studies that looked at women who raised their own biological children, those who didn't smoke during pregnancy had kids who scored about a 99, on average, compared to a score of 104 among children whose mothers smoked 10 or more cigarettes per day.
That difference, according to Harold, would be noticeable in daily life.
Similar results were observed for children who were raised by adoptive mothers when researchers surveyed their birth mothers about smoking.
While the new study cannot prove smoking in pregnancy caused the behaviour problems, Harold told Reuters Health it helps rule out some other potential explanations.
"It's illuminating the prenatal period as having an ongoing influence on outcomes," Harold said.
"We're not saying life after birth is no longer relevant… Rather, both influences are clearly important," he said.
Harold said there are a few explanations for why smoking may influence later conduct, including that babies of mothers who smoke may be born smaller or have impaired brain development.
In an accompanying editorial, Theodore Slotkin, a professor of pharmacology and cancer biology at the Duke University School of Medicine in Durham, North Carolina, writes that it's undeniable that smoking while pregnant contributes to later behaviour problems in children, based on the new study and past research.
He also told Reuters Health the concern goes beyond women's own smoking.
"It's one thing if mom is smoking, but what if mom is getting exposed from a partner or a smoking environment?" Slotkin said.
Harold said the new findings add to the reasons for women not to smoke while pregnant.
"Providing a healthy and safe prenatal environment, giving that child the best possible starting place, helps the child in the long term," he said.