Smoking tobacco during pregnancy has well-documented negative effects on infants’ birth weight and is linked to several childhood health problems. Now, research from the University at Buffalo has found that smoking marijuana during pregnancy can also affect infants’ weight and behavior problems, especially when combined with tobacco use.

Rina Das Eiden, a senior research scientist at the university’s Research Institute on Addictions, studied nearly 250 infants and their mothers. Of these, 173 of the infants had been exposed to tobacco and/or marijuana during their mothers’ pregnancies. None were exposed to significant amounts of alcohol.

Eiden 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, and were more likely to be born earlier, compared to babies who were not exposed to anything. What suggests marijuana may also have a detrimental effect on fetal development is the finding that infants exposed to both tobacco and marijuana were also more likely to be smaller in length and weight compared to babies exposed only to tobacco in the third trimester. The results were stronger for boys compared to girls.

“We also found that lower birth weight and size predicted a baby’s behavior in later infancy,” Eiden says. “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 behavior via poor growth at delivery.”

Women who showed symptoms of anger, hostility and aggression reported more stress in pregnancy and were more likely to continue using tobacco and/or marijuana throughout pregnancy. Other research has linked infants’ irritability and frustration to higher levels of maternal stress during pregnancy, suggesting a triple threat to fetal development from maternal stress, tobacco exposure and marijuana exposure.

“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 says. “This may help reduce prenatal substance exposure and subsequent behavior problems in infants.”

This means that for women who enjoy the occasional joint, during pregnancy it’s best to find a new method of indulgence. But just think how good that first joint will feel after nine months (well, maybe a little longer if you breastfeed).