Up to 25 percent of child behaviour problems may be attributed to mothers smoking in early pregnancy, according to a University of Queensland study.
It also found that maternal smoking may account for a further 16 percent of behaviour problems in children around age five.
The study was led by Associate Professor Gail Williams from the University's Australian Centre for International and Tropical Health and Nutrition, Dr Michael O'Callaghan of the Mater Children's Hospital, and University Anthropology and Sociology Department head Professor Jake Najman.
Recently published in the prestigious journal Paediatrics, this is the first known longitudinal study to examine the association between child behaviour problems such as aggression and delinquency and smoking before, during and after pregnancy. Data were taken from the Mater Hospital-University of Queensland Study of Pregnancy.
Professor Najman said the study provided further evidence for anti-smoking campaigns targeting pregnant women.
The study examined 4879 five-year-old children whose mothers were recruited early in pregnancy. Smoking history was gathered for pre-pregnancy, first clinic visit, late pregnancy and when the child was six months old and aged five.
Before pregnancy, 46.5 percent of the group were smokers. This declined to 35.4 percent at the first clinic visit and 37.3 percent at the five-year follow-up.
In order to test the link between smoking and child behaviour problems, the researchers eliminated as many other alternative explanations as possible such as the mother's age at time of birth, education level, marital status, mental health, employment status, the child's gender and presence of siblings.
Following this analysis, they found the direct link between maternal smoking, especially in early pregnancy, remained.
Child behaviour problems were measured using 33 Achenbach and Edelbrock standardised descriptions of problems for this age group.
Smoking at the first clinic visit rather than at other times had the greatest association with later child behaviour problems suggesting a biological rather than a social reason, Dr O'Callaghan said.
"It's possible nicotine derived from the mother's smoking could alter gene expression and the function of nicotine receptors in the foetal brain," he said.
Further studies were needed to establish the exact reason for the association between smoking in early pregnancy and high rates of child behaviour problems, he said.
A second (weaker) association was between maternal smoking five years after the birth and child behaviour problems when the child was five. Both biological and social reasons could be to blame including receiving nicotine through passive smoking.
The researchers compared mothers' smoking patterns with the percentages of children developing behaviour problems.
Non-smoking mothers had a low risk of child behaviour problems (8.2 percent) compared with mothers who smoked at all points (14.7 percent). If women stopped smoking at the first clinic visit, the risk of child behaviour problems was reduced to around that of continuous non-smokers (7.9 percent).
Other researchers involved with the study were: Margaret Andersen from the University's Obstetrics and Gynaecology Department; David Richards and Chinlyn U from the Academic Division of Child Health, School of Human Development, University of Nottingham (UK); and Dr William Bor from the Enoggera Child and Youth Mental Health Service.
For more information, contact Professor Najman (telephone 07 3365 3152) or Dr Williams (telephone 07 3365 5406 or mobile 0412 168 914).
