"The failure to find an association between prenatal marijuana exposure and a variety of cognitive outcomes persisted in the OPPS sample until the offspring were 4 years of age. At 2, although there was a negative association with language comprehension, this relationship did not retain significance when the home environment was statistically controlled (Fried and Watkinson 1988). At 3 years of age, after controlling for confounding factors, prenatal marijuana exposure was not associated with language expression and comprehension or decreased cognitive scores (Fried and Watkinson 1990)."
Peter A. Fried PhD (2002) The Consequences of Marijuana Use During Pregnancy: A Review of the Human Literature, Journal of Cannabis Therapeutics, 2:3-4, 85-104, DOI: 10.1300/J175v02n03_06