Cigarettes, Alcohol and Marijuana: Varying Associations with Birthweight

Jennie Kline, Zena Stein and Michelle Hutzler

The relations of cigarette smoking, alcohol drinking and marijuana use during pregnancy to birthweight were examined in two prospectively studied pregnancy cohorts (Phases I and II). After analytic adjustment in ordinary least squares regressions for other factors that influence birthweight, cigarette smoking during at least half the pregnancy was associated with a significant decrease in mean birthweight (159 grams in Phase I, 202 grams in Phase II) In Phase II only, beer drinking was associated with a significant decrement of 8.4 grams in estimated birthweight per ounce of absolute alcohol per month. Neither wine nor liquor drinking in the Phase II data nor any of the three beverages in the Phase I data was associated with significant decrements in predicted birthweight. Furthermore, with one exception (drinking once a week in Phase II only), alcohol drinking, defined as the number of occasions per month on which any alcoholic beverage was consumed, was not associated with a change in birthweight. Regarding marijuana use, the data are not consistent between the two phases. In the Phase I data, no coherent trend in association with birthweight was observed. In the Phase II data, marijuana use 2-3 times per week, 4-6 times per week and daily was associated with increasing decrements in estimated birthweight: 127 g, 143 g, and 230g respectively. The inconsistent findings for alcohol drinking and marijuana use between phases stand in marked contrast to the consistent findings for cigarette smoking. The evidence regarding marijuana use and birthweight is sparse, with one study reporting a positive association (15) and another no association. (16)

The greatest degree of inconsistency between the two phases was observed for daily use of marijuana. In Phase I, use 2-4 times per month was associated with a significant increase in birthweight; in Phase II, daily use was associated with a decrease in birthweight, with lesser decrements for use 2-3 and 4-6 times per week. The questions asked in the two phases were not directly comparable, although one would have thought the modification too slight to explain the differences. The proportion of daily users is higher in Phase II, an observation that might have resulted from the change in the questionnaire. One suggestion is that the composition of the substance has changed, or perhaps that it was subjected to a contaiminant or herbicide over the second phase. Certainly marijuana, unlike the other two exposures, is unstandardized and likely to vary. It is also possible that the amount smoked on each occasion could be higher in Phase II than in Phase I, but we have no information on this point.

Finally, we thought that marijuana users in more recent times might be taking cocaine or other drugs as well as marijuana. This proved to be the case, but among women not using cocaine, a decrement in birthweight of borderline significance persists for Phase II women who smoke marijuana daily.

(15) Hingson R, et al: Effects of maternal drinking and marijuana use on fetal growth and development. "Pediatrics" 1982; 70: 539-46

(16) Linn S, Schoenbaum S C, et al: The association of marijuana use with outcome of pregnancy. AJPH 1983; 73: 1161-4.