Mother-to-child transmission of human immunodeficiency virus type 1 (HIV-1) often occurs during labor or at the time of delivery. Suspected mechanisms are direct contact of the fetus with infected blood or secretions, especially after rupture of membranes, and the transfusion of maternal blood to the fetus during labor. Thus, the idea that delivery by cesarean section before the onset of labor or rupture of membranes may reduce the risk of vertical transmission of HIV has been the focus of studies. The International Perinatal HIV Group performed a meta-analysis using data on persons from 15 prospective cohort studies to evaluate the association between the mode of delivery and the risk of mother-to-infant transmission of the virus.

Studies selected for the meta-analysis contained at least 100 mother-child pairs. Required data included mode of delivery, the child's infection status and absence of breast-feeding. All were English-language studies. The four categories of delivery included: non-instrumental vaginal delivery; instrumental vaginal delivery (vacuum, forceps or both); elective cesarean section performed before the onset of labor and rupture of membranes; and nonelective cesarean section performed after onset of labor or rupture of membranes. Potential covariates of transmission risk included in the analysis were based on previously published data. These included receipt of antiviral therapy, having an AIDS diagnosis, substance abuse during pregnancy and maternal CD4 lymphocyte counts.

Fifteen (five European and 10 North American) of 16 eligible studies were included in the meta-analysis and contained data from 1982 through 1996. Of the 15,471 mother-child pairs, 8,533 were included in the study after exclusion criteria were applied. The use of antiviral therapy markedly increased in 1994 after data from AIDS Clinical Trials Group Protocol 076 showed that a three-part regimen of zidovudine given to the mother before and during labor and delivery, and to the infant after birth, dramatically reduced HIV transmission. A logistic-regression model, applied to 7,840 mother-infant pairs and adjusted for maternal stage of disease, use of antiviral therapy and low birth weight of infants revealed that elective cesarean section was strongly associated with a lower risk of vertical transmission compared with all other modes of delivery. The crude rate of vertical transmission with elective cesarean section was 8.4 percent, compared with 11.7 percent if performed within one hour of rupture of membranes, and 13.5 percent if performed within four hours of rupture of membranes. In mothers who underwent elective cesarean section and received antiviral therapy during the prenatal, intrapartum and neonatal periods, the transmission rate was only 2 percent with elective cesarean section compared with 7.3 percent for the other three modes of delivery.

The authors conclude that elective cesarean section significantly lowers the risk of vertical transmission of HIV-1. When combined with the use of zidovudine, the risk is decreased even further than the risk among women in whom only one of these interventions is used. All HIV-infected women who become pregnant should be advised of the potential benefits of elective cesarean section as well as the potential risks of this surgical procedure.

editor's note: This study has obvious significant national and international implications. The journal actually released this data via a Web site almost two months in advance of print publication because of the public health implications. However, as Riley and Greene point out in an accompanying editorial, an important limitation of this study is a lack of information on maternal HIV-RNA levels and of data on women being treated with combination antiviral therapy, which has now become the standard of care. If a mother is taking combination therapy and has a nonde-tectable viral load, the risk of vertical transmission is probably very low regardless of the mode of delivery. Whether having a cesarean section can further reduce the risk will be difficult to prove. Clinicians providing prenatal care to HIV-infected mothers are obligated to present the available data concerning vertical transmission of HIV-1 and to consider the risks and the benefits of cesarean delivery. This is not an easy task.—j.k.