"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)."