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Objective To document the prevalence of cannabis use in a large sample of British women studied during pregnancy, to determine the association between cannabis use and social and lifestyle factors and assess any independent effects on pregnancy outcome.

Design Self-completed questionnaire on use of cannabis before and during pregnancy.

Sample Over 12,000 women expecting singletons at 18 to 20 weeks of gestation who were enrolled in the Avon Longitudinal Study of Pregnancy and Childhood.

Methods Any association with the use of cannabis before and during pregnancy with pregnancy outcome was examined, taking into account potentially confounding factors including maternal social background and other substance use during pregnancy.

Main outcome measures Late fetal and perinatal death, special care admission of the newborn infant, birthweight, birth length and head circumference.

Results Five percent of mothers reported smoking cannabis before and/or during pregnancy; they were younger, of lower parity, better educated and more likely to use alcohol, cigarettes, coffee, tea and hard drugs. Cannabis use during pregnancy was unrelated to risk of perinatal death or need for special care, but, the babies of women who used cannabis at least once per week before and throughout pregnancy were 216g lighter than those of non-users, had significantly shorter birth lengths and smaller head circumferences. After adjustment for confounding factors, the association between cannabis use and birthweight failed to be statistically significant (P= 0.056) and was clearly non-linear: the adjusted mean birthweights for babies of women using cannabis at least once per week before and throughout pregnancy were 90g lighter than the offspring of other women. No significant adjusted effects were seen for birth length and head circumference.

Conclusions The results of this study suggest that the use of cannabis during pregnancy was not associated with increased risk of perinatal mortality or morbidity in this sample. However, frequent and regular use of cannabis throughout pregnancy may be associated with small but statistically detectable decrements in birthweight.