What do we know about marijuana use among women of reproductive age and about the use of marijuana during pregnancy? Marijuana is the most commonly used illicit substance, and after alcohol and tobacco, the most commonly used drug during pregnancy. Women who use marijuana are more likely to be white, younger, and to use other substances. The characteristics of women who use marijuana during early pregnancy are similar, although women who continue to use marijuana throughout pregnancy are somewhat different. These women are less-well educated, of lower social class, much more likely to use other substances, and more likely to be black. We do not know why some women use marijuana while others do not, and why some women discontinue their use during pregnancy while others do not. What do we know about the effects of marijuana use during pregnancy? A number of studies have investigated the relationship between prenatal marijuana exposure and outcome at birth. The results, unfortunately, are equivocal. Prospective studies that have examined women at regular and frequent intervals during pregnancy, in general, have not found a relationship between marijuana use and birthweight (Day NL, Sambamoorthi U, Taylor P, et al: unpublished data, 1990) although some have reported a small effect of marijuana use on birth length (Day NL, Sambamoorthi U, Taylor P, et al: unpublished data, 1990). Other studies, some prospective and some retrospective, have reported correlations between marijuana use during pregnancy and smaller size at birth. Several of these studies, however, failed to control adequately for other illicit drug use while one used marijuana only as a dichotomous variable in the analysis. Therefore, we do not yet know whether there is or is not an effect of marijuana use during pregnancy on intrauterine growth retardation. Only a few studies have reported on growth outside the neonatal period, and these studies have not found a consistent effect of prenatal marijuana exposure. There are, however, too few reports to assume that this is definitive. Several studies reported a relationship between prenatal marijuana use and the gestational age of the infant. As with growth, however, other studies have not corroborated these findings. Similarly, two studies have noted an increase in morphologic abnormalities, although one of these did not have a control group for comparison. Most studies have reported finding no relationship with either minor or major morphologic abnormalities. At birth, investigators have assessed the relationship between prenatal marijuana exposure and neurobehavioral outcome. Again, the results are contradictory.(ABSTRACT TRUNCATED AT 400 WORDS)