Use of Marijuana During Pregnancy By Lynn Zimmer and John P. Morgan

Warnings that marijuana causes birth defects date back to the late 1960s.1 Some researchers claimed to have found chromosomal abnormalities in blood cells taken from marijuana users. They predicted that young men and women who used marijuana would produce deformed babies.2 Although later studies disproved this theory,3 some current drug education materials still claim that genetic damage is passed on by marijuana users to their children.4

Today, researchers look for a direct effect of THC [for tetrahydrocannabinol, either of two physiologically active isomers, C21H30O2, from hemp plant resin] on the fetus. In animal studies, THC has been shown to produce spontaneous abortion, low birth weight, and physical deformities—but only with extremely large doses, only in some species of rodents, and only when THC is given at specific times during pregnancy.5 Because the effects of drugs on fetal development differ substantially across species,6 these studies have little or no relevance to humans. Studies with primates show little evidence of fetal harm from THC.7 In one study, researchers exposed chimpanzees to high doses of THC for up to 152 days and found no change in the sexual behavior, fertility, or health of their offspring.8

Dozens of studies have compared the newborn babies of women who used marijuana during pregnancy with the babies of women who did not. Mainly, they have looked for differences in birth weight, birth length, head circumference, chest circumference, gestational age, neurological development, and physical abnormalities. Most of these studies, including the largest study to date with a sample of over twelve thousand women,9 have found no differences between babies exposed to marijuana prenatally and babies not exposed.10 Given the large number of studies and the large number of measures, some differences are likely to occur by chance. Indeed, researchers have found differences in both directions. In some studies, the babies of marijuana users appear healthier and hardier.11 In others, researchers have found more adverse outcomes in the babies of marijuana users.12

When adverse outcomes are found, they are inconsistent from one study to another, always relatively minor, and appear to have no impact on infant health or mortality.13 For example, in one recent study, researchers reported a statistically significant effect of marijuana on birth length. The marijuana-exposed babies, on average, were less than two-tenths of one inch shorter than babies not exposed to marijuana.14 Another study found a negative effect of marijuana on birth weight, but only for White women in the sample.15 In a third study, marijuana exposure had no effect on birth weight, but a small negative effect on gestational age.16 Overall, this research indicates no adverse effect of prenatal marijuana exposure on the physical health of newborns.

Researchers have also examined older children for the effects of prenatal exposure to marijuana. A study of one-year-olds found no differences between marijuana-exposed and nonexposed babies on measures of health, temperament, personality, sleeping patterns, eating habits, psychomotor ability, physical development, or mental functioning.17 In two studies, one of three-year-olds,18 the other of four-year-olds,19 there was no effect of prenatal marijuana exposure on children’s overall IQ test scores. However, in the first study, when researches looked at Black and White children separately, they found, among Black children only, slightly lower scores on two subscales of the IQ test. On one subscale, it was children exposed to marijuana only during the first trimester who scored lower. On the other subscale, it was children exposed during the second trimester who scored lower.20 In neither case did the frequency or quantity of mothers’ marijuana use affect the outcomes. This makes it highly unlikely they were actually caused by marijuana. Nonetheless, this study is now cited as evidence that using marijuana during pregnancy impairs the intellectual capacity of children.21

Also widely cited are two recent case-control studies describing a relationship between marijuana use by pregnant women and two rare forms of cancer in their children. A case-control study compares people with a specific disease (the case sample) to people without the disease (the control sample). Using this method, researchers identify group differences in background, environment, lifestyle, drug use, diet, and the like that are possible causes of the disease.

A study of children with non-lymphoblastic leukemia reported a tenfold greater risk related to their mothers’ use of marijuana during pregnancy.22 A second study reported a threefold greater risk of rhabdomyosarcoma.23 These calculations were based on women’s reports that they used marijuana at some point during pregnancy. In the first study, ten out of the 204 case-group mothers (5 percent) reported marijuana use, compared to one out of the 204 control-group mothers (0.5 percent). In the second study, 8 percent of case-group mothers reported using marijuana, compared to 4.3 percent of controls.