Mothers who smoke during pregnancy put their children at greater risk of developing psychotic symptoms as teenagers, British scientists said on Thursday.

Researchers from four British universities studied 6,356 12-year-olds and interviewed them for psychotic-like symptoms such as hallucinations or delusions. Around 19 per cent had mothers who smoked during pregnancy.

Just over 11 per cent, or 734 of the total group, had suspected or definite symptoms of psychosis.

Many previous studies have shown cigarettes can harm the fetuses of mothers who smoke while pregnant. The risks include causing babies to be born smaller and increasing the risk of sudden infant death syndrome or heart defects.

Stanley Zammit, a psychiatrist at Cardiff University's School of Medicine who led the study, said the more the mothers smoked, the more likely their children were to have psychotic symptoms.

"We can estimate that about 20 per cent of adolescents in this cohort would not have developed psychotic symptoms if their mothers had not smoked," he said.

Despite countless studies flagging up the risks to babies, it is estimated that between 15 and 20 per cent of women in Britain smoke during pregnancy.

The researchers also found drinking during pregnancy was associated with increased psychotic symptoms, but only in children whose mothers had drunk more than 21 units of alcohol a week in early pregnancy.

The reasons for the link between maternal smoking and psychotic symptoms are not clear, but Zammit and colleagues suggested that exposure to tobacco in the womb might affect a child's impulsivity, attention or cognition.

They said more research was needed to investigate how exposure to tobacco in the womb affected children's brains.

Only a few mothers in the study, which was published in the British Journal of Psychiatry, said they had smoked cannabis during pregnancy, and this was not found to have any significant link with psychotic symptoms. – Reuters