It has been well documented that smoking during pregnancy can harm the developing baby, but now, according to researchers, women who smoke during pregnancy may increase the risk of their child displaying anti-social behaviour.
They say there is a "small but significant" link between maternal smoking and both unruly behaviour and attention deficit hyperactivity disorder.
In a study of 1,896 twins it was found that the average symptom scores for both increased with the number of cigarettes the mother had smoked while pregnant.
The researchers at the Institute of Psychiatry say the findings do not mean unruly behaviour and attention deficit hyperactivity disorder (ADHD) were linked, although ADHD is known to increase the risk of anti-social behaviour.
In previous studies maternal smoking has been linked to both ADHD and anti-social behaviour, but it was unclear whether the increased risk of anti-social behaviour was linked to the ADHD rather than maternal smoking per se.
ADHD is a serious behavioural disorder which experts estimate may affect up to 6 percent of children, and people with the condition have a poor attention span and tend to be impulsive and restless.
It is known to increase the likelihood of anti-social behaviour.
Although ADHD is thought to have a strong medical element, social factors are often blamed for unruly behaviour.
However this new study suggests a biological cause for the anti-social behaviour.
The team at the Institute of Psychiatry, in London, sent questionnaires to the parents of 723 identical twins and 1,173 non-identical twins.
The parents were asked to provide information on their own smoking habits and on their children's behaviour.
A third of the mothers said they had smoked during pregnancy.
Behaviour was considered anti-social if the child bullied others, often destroyed his or her own or others' belongings, had a habit of stealing things, often told lies or was frequently disobedient.
It was found overall, that a small minority of the children had anti-social behaviour or ADHD - 4-11 percent, but when the researchers looked at the influence of maternal smoking, they found it contributed in a small but significant way to both disorders.
The team found that the average symptom scores for both anti-social behaviour and ADHD increased with the number of cigarettes smoked by the mother.
The were apparently several explanations for the trend, including direct effects of tobacco smoke on the developing baby.
Lead researcher Dr Tanya Button says the most commonly cited one is the influence of nicotine on brain development in the foetus, possibly leading to neurological impairment.
Maternal prenatal smoking can also reduce the amount of oxygen getting to the foetus, which may also be detrimental to later behavioural outcomes.
Although there may be other factors playing a far bigger part in both conditions, the researchers advise any woman who is pregnant to avoid smoking.
Professor Eric Taylor, also from London's Institute of Psychiatry, but independent from the research, says showing that maternal prenatal smoking can have a biological influence on conduct disorder, is useful to know.
Professor Taylor, who is also an expert for the ADHD charity ADDISS, agrees that pregnant women should not smoke for many health reasons.
He suggests the effects are probably dose-related and the more you smoke the greater the risk is, and it is not just mediated through the personality of the mother or social disadvantage, it has a direct effect on developing babies.
He also thought it was possible that the mothers were transmitting "anti-social behaviour" genes to their children.
The study is published in the British Journal of Psychiatry.
http://bjp.rcpsych.org/ and http://www.iop.kcl.ac.uk/