Sexual behavior and injection drug use during pregnancy and vertical transmission of HIV-1

We evaluated maternal sexual behavior and injection drug use practices as possible risk factors for vertical transmission of human immunodeficiency virus type 1 (HIV-1). Data were analyzed from the Mothers and Infants Cohort Study, a prospective study in Brooklyn and the Bronx, New York. A total of 207 mother-infant sets were enrolled between 1986 and 1991 and followed for up to 4 years after the enrollment visit during pregnancy. HIV-1 transmission occurred in 49 of 201 mother-infant sets, yielding an overall transmission rate of 24.4% (95% confidence interval (CI) = 18.7% to 31.0%). Increased frequency of vaginal intercourse after the first trimester of pregnancy was positively associated with vertical transmission of HIV-1 (trend p = 0.03). A lifetime history of injection drug use was not associated with vertical transmission. However, a history of combined cocaine and heroin injection after the first trimester of pregnancy was associated with vertical HIV-1 transmission, particularly among women with CD4⁺ lymphocyte levels of 20% or higher (risk ratio = 4.0; 95% CI = 2.0 to 8.1). Cocaine and heroin injection drug use after the first trimester accounted for most of the relation between preterm birth and vertical HIV-1 transmission in this cohort. Maternal coinfection with hepatitis C virus or human T-cell lymphotropic virus types I and II could not explain these observations, because coinfection with these viruses had no detectable effect on HIV-1 transmission. These results suggest that maternal sexual behavior and injection drug use practices during the second and third trimester of pregnancy may modify the risk of vertical HIV- 1 transmission.