Substance abuse during pregnancy: effect on pregnancy outcomes

To determine the contribution of drug use to maternal and perinatal complications, controlling for social confounders.

This is a retrospective cohort study of 247 drug-using women and 741 controls over a 4-year period from 1997 to 2000. Cases were identified from the drug dependency register. Three controls for each woman with substance abuse were selected from the delivery suite records, with calliper matching by year of delivery (any control patient who delivered within 6 months before or after the date of delivery of a drug-using woman was considered as a potential match) and district of residence (post code). The primary outcomes of interest were preterm birth, abruption, pre-eclampsia, intrauterine growth restriction and low birth weight.

There were statistically significantly more preterm births amongst drug-using women (relative risk (RR) 2.5, 95% confidence interval (CI) 1.6-3.8), with preterm births complicating 25% of births amongst drug users. The incidence of low birth weight was 30.8% amongst drug-using women compared to 8% in control women (RR 3.6, CI 2.4-5.4), and the incidence of growth restriction was 25%, significantly higher than the control group (RR 3.82, CI 2.4-6.1). The risk of abruption was also higher (RR 2.74, CI 1.1-7.0). Of note is the extremely low incidence of pre-eclampsia among drug users, even after controlling for the confounder effects of parity and smoking.

Despite multidisciplinary co-ordinated antenatal care, women with substance abuse during pregnancy are at significant risk of adverse obstetric and perinatal outcome, controlling for social confounders. A limitation of the study is that the sample size was not large enough to clearly assess individual drugs. This is the first study to highlight low incidence of pre-eclampsia among drug users over and above the effect of smoking. Further research is needed to elucidate the underlying biological reason for the lack of pre-eclampsia in women with substance abuse during pregnancy.