After finding a slight deficit in visual responsiveness among marijuana-exposed newborns, no differences were found at six months, 12 months, 18 months, or 24 months. At age 3, the only difference (after controlling for confounding variables) was that children of "moderate" smokers had superior psycho-motor skills. At age 4, children of "heavy" marijuana users (averaging 18.7 joints/week) had lower scores on one subscale of one standardized test of verbal development. At age 6, these same children scored lower on one computerized task - that measuring "vigilance." On dozens of others scales and subscales, no differences were ever found.

In another study, standardized IQ tests were administered to marijuana-exposed and unexposed three year-olds. Researchers found no differences in the overall scores. However, by dividing the sample by race, they found - among African-American children only - lower scores on one subscale for those exposed during the first trimester and lower scores on a different subscale for those exposed during the second trimester.

http://www.erowid.org/plants/cannabi...is_myth7.shtml