Women who smoke marijuana or use cocaine while they are pregnant are more likely to give birth to smaller babies, increasing the infants' chances of serious health problems, researchers reported this week.

In the largest study of its kind and the first to use urine testing, researchers found women who tested positively for marijuana or cocaine use while they were pregnant tended to have lighter, shorter babies with smaller heads."I would argue it's the best study on both (marijuana and cocaine use) for different reasons," said Dr. Barry Zuckerman, head of the study and professor of pediatrics at Boston University medical school.

"The take-home message is don't smoke marijuana or use cocaine during pregnancy," Zuckerman said.

The study also indicates cocaine and marijuana use along with other unhealthy lifestyle habits, such as cigarette smoking and poor nutrition, combine to increase the risk of giving birth to underdeveloped babies, he said.

Based on the study, the researchers calculated a woman who gained only 10 pounds while she was pregnant, smoked one pack of cigarettes a day and used cocaine and marijuana could be expected to give birth to a baby that weighed about 1 pound less than a women who did none of those things.

"Women should be advised that marijuana use and cocaine use during pregnancy are each independently associated with impaired fetal growth. Moreover, as compared with women who do not use these substances, women who do use them during pregnancy are more likely to have other factors, such as cigarette smoking and decreased weight gain, that are associated with depressed fetal growth," the researchers wrote.

Previous studies had indicated using marijuana or cocaine during pregnancy could be dangerous for the fetus. But previous marijuana studies had produced mixed results and studies examining the effects of cocaine use involved women who might not have been representative of the general population, Zuckerman said.

The new study, published in The New England Journal of Medicine, is the first to examine marijuana use by testing pregnant women's urine for the drug and the first to test the effect of cocaine use in a representative sample of women, he said.

The researchers studied 1,226 women who sought prenatal care at the Boston City Hospital from 1984 to 1987. The women were questioned about a variety of aspects of their lifestyles and underwent urine testing while they were pregnant or just after they gave birth.

When all other factors were taken into consideration, the women who tested positively for marijuana use gave birth to babies who were about 3 ounces lighter on average than those delivered from mothers who tested negatively for marijuana use.