New York, May 11 (IANS): If you smoke marijuana during your pregnancy, your baby could be at high risk of being born with a low birth weight as well as influencing behaviour problems, researchers have warned.

The findings, published in the journal Child Development, suggests that prenatal marijuana use can have consequences on infant's weight and can influence behaviour problems, especially when combined with tobacco use.

The researchers found that infants who had been exposed to both tobacco and marijuana, especially into the third trimester, were smaller in length, weight and head size.

They were more likely to be born earlier, compared to babies who were not exposed to anything, the researcher said.

"We also found that lower birth weight and size predicted a baby's behaviour in later infancy," said co-author Rina Das Eiden from the University at Buffalo in New York.

"Babies who were smaller were reported by their mothers to be more irritable, more easily frustrated and had greater difficulty calming themselves when frustrated. Thus, there was an indirect association between co-exposure to tobacco and marijuana and infant behaviour via poor growth at delivery," Eiden added.

For the study, the researchers recruited nearly 250 infants and their mothers. Of these, 173 of the infants had been exposed to tobacco or marijuana during their mothers' pregnancies. None were exposed to significant amounts of alcohol.

Women who showed symptoms of anger, hostility and aggression reported more stress during pregnancy and were more likely to continue using tobacco and marijuana throughout pregnancy, the researcher said.

Therefore, due to the co-exposure, they were more likely to give birth to infants smaller in size and who were more irritable and easily frustrated, the researchers added.

The infants' irritability and frustration is also linked to mothers who experienced higher levels of stress while pregnant.

"Our results suggest that interventions with women who smoke cigarettes or use marijuana while pregnant should also focus on reducing stress and helping them cope with negative emotions," Eiden said.

"This may help reduce prenatal substance exposure and subsequent behaviour problems in infants," Eiden noted.