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Medical References
Exposing Marijuana Myths

Marijuana Myths, Claim No. 7



A powerful accusation in anti-drug campaigns is that children are permanently harmed by their mothers' use of drugs during pregnancy. Today, it is commonly claimed that marijuana is a cause of birth defects and development deficits.


A number of studies claimed reported low birth weight and physical abnormalities among babies exposed to marijuana in utero. 43 However, when other factors known to affect pregnancy outcomes were controlled for - for example, maternal age, socioeconomic class, and alcohol and tobacco use - the association between marijuana use and adverse fetal effects disappeared. 44

Numerous other studies have failed to find negative impacts from marijuana exposure. 45 However, when negative outcomes are found, they tend to be widely publicized, regardless of the quality of the study.

It is now often claimed that marijuana use during pregnancy causes childhood leukemia. The basis for this claim is one study, in which . 5% of the mothers of leukemic children admitted to using marijuana prior to or during pregnancy. A "control group" of mothers with normal children was then created and questioned by telephone about previous drug use. Their reported .5 % marijuana use-rate was used to calculate a 10-fold greater risk of leukemia for children born to marijuana users. 46 Given national surveys showing marijuana prevalence rates of at least 10%, these "control group" mothers almost certainly under-reported their drug use to strangers on the telephone.
Also used as evidence of marijuana-induced fetal harm are two longitudinal studies, in which the children of marijuana users were examined repeatedly. However, on closer examination, the effects of marijuana appear to be quite minimal, if existent at all.
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. 47 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. 48 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. 49

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. 50

Although it is sensible to advise pregnant women to abstain from using most drugs - including marijuana - the weight of scientific evidence indicates that marijuana has few adverse consequences for the developing human fetus.

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