MATERNAL COCAINE USE AND LOW-BIRTH-WEIGHT NEWBORNS - A METAANALYSIS

Aim/design. Many epidemiological studies published on the association between maternal cocaine/crack we and birth weight have either lacked precision or failed to control for major confounding, predominantly by tobacco smoking. Meta-analysis enables a single summary measure of ef fect to be calculated by combining data from any number of individual studies, thus enhancing statistical power. We undertook a number of me ta-analyses using only studies that had adjusted for tobacco smoking t o estimate more precisely the effect of maternal cocaine we on birth w eight. Findings. A meta-analysis of five studies presenting data for ' any' prenatal cocaine exposure, adjusted for tobacco smoking but unadj usted for gestational age, produced a pooled relative risk estimate fr om a fixed effects analysis of 2.15 (95% CI 1.75-2.64). However, there was substantial heterogeneity among studies (p < 0.001), and the rela tive risk from a random effects analysis was smaller (1.65) with a con fidence interval that included unity (95% CI 0.94-2.83). Addition of a further study adjusted for gestational age had minimal effect on the pooled estimate: the fixed effects relative risk was 2.14 (1.77-2.60) and the random effects estimate 1.77 (1.15-2.71). When data on more in tense prenatal exposure were analysed, the fixed and random effects an alysis produced the same pooled estimate of the relative risk of 4.42 (2.24-8.71), suggesting that more frequent cocaine exposure was associ ated with a higher relative risk for low birth weight. Data from studi es on mean reduction in birth weight produced a pooled estimate of 112 g (95% CI 62-161 g). Conclusions. The current study suggests that mat ernal cocaine use causes low birth weight, and that the effect is grea ter with heavier use. However, despite the adjustment for tobacco and the adjustment by some studies for other confounders such as race, mat ernal age, gravidity and socio-economic status, it could be argued tha t other life-style factors not controlled for may account for the obse rved effects. While this argument is not supported by some other types of study, the issue of residual confounding can only be finally addre ssed by analytical studies which adequately control for important vari ables.