Institute of Medicine

Among the studies that have investigated the relationship between prenatal marijuana exposure and birth outcome, the results have been inconsistent. Except for adolescent mothers, there is little evidence that gestation is shorter in mothers who smoke marijuana. Several studies of women who smoked marijuana regularly during pregnancy show that they tend to give birth to lower weight babies. (...)
For most of these studies, much of the harms associated with marijuana use are consistent with those associated with tobacco use, and smoking is a significant factor so the contribution of cannabinoids cannot be confirmed. However, Jamaican women who use marijuana rarely smoke it, but instead prepare it as tea. In a study of neonates born to Jamaican women who either did or did not ingest marijuana during pregnancy, there was no difference in neurobehavioral assessments made at 3 days after birth and at one month. (...)
Since 1978, the Ottawa Prenatal Prospective Study has been measuring the cognitive functions of children born to mothers who smoked marijuana during pregnancy. (...) The children in the different marijuana exposure groups showed no lasting differences in global measures of intelligence such as language development, reading scores, and visual or perceptual tests. Moderate cognitive deficits were detectable among these children when they were four days old and again at four years, but these deficits were no longer apparent at five years. Prenatal marijuana exposure was not, however, without lasting impact. By comparison, at both ages 5-6 and 9-12, children in the same study who were prenatally exposed to tobacco smoke scored significantly lower on tests of language skills and cognitive functioning.
Joy JE, Watson SJ, Benson JA, eds. Marijuana and medicine: Assessing the science base. Institute of Medicine. Washington DC: National Academy Press, 1999.