Background. A substantial proportion of perinatally acquired infections with the human immunodeficiency virus type 1 (HIV-1) occur at or near delivery, which suggests that obstetrical factors may have an important influence on transmission. We evaluated the relation of such factors and other variables to the perinatal transmission of HIV-1. Methods. The Women and Infants Transmission Study is a prospective, observational study of HIV-1-infected women who were enrolled during pregnancy and followed with their infants for three years after delivery. We studied obstetrical, clinical, immunologic, and virologic data on 525 women who delivered live singleton infants whose HIV-1-infection status was known as of August 31, 1994. Results. Among mothers with membranes that ruptured more than four hours before delivery, the rate of transmission of HIV-1 to the infants was 25 percent, as compared with 14 percent among mothers with membranes that ruptured four hours or less before delivery. In a multivariate analysis, the presence of ruptured membranes for more than four hours nearly doubled the risk of transmission (odds ratio, 1.82; 95 percent confidence interval, 1.10 to 3.00; P=0.02), regardless of the mode of delivery. The other maternal factors independently associated with transmission were illicit-drug use during pregnancy (odds ratio, 1.90; 95 percent confidence interval, 1.14 to 3.16; P= 0.01), low antenatal CD4+ lymphocyte count (<29 percent of total lymphocytes) (odds ratio, 2.82; 1.67 to 4.76; P<0.001), and birth weight <2500 g (odds ratio, 1.86; 1.03 to 3.34; P=0.04). Conclusions. The risk of transmission of HIV-1 from mother to infant increases when the fetal membranes rupture more than four hours before delivery.
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