For every hour that elapses after an HIV-infected pregnant woman's membranes rupture, the risk that the virus will be transmitted to her offspring rises by 2%, according to a meta-analysis involving data on nearly 5,000 deliveries.1 Moreover, among women with AIDS, the risk of transmission increases steeply as the time since rupture of the membranes grows--from 8% at two hours to 31% at 24 hours.
To test the a priori hypothesis that the longer the membranes have been ruptured, the higher the risk of vertical transmission, the researchers pooled data from 15 prospective cohort studies conducted in Europe and North America. They included only deliveries (vaginal or cesarean) that occurred after the onset of labor or within 24 hours after the membranes ruptured; cesarean sections performed before labor and before rupture of membranes were excluded. The analyses are based on 4,721 deliveries, which occurred between 1982 and 1990.
In univariate analyses, the duration of ruptured membranes was significantly associated with the risk of transmission. Twelve percent of women whose membranes were ruptured for less than one hour transmitted HIV to their infants, and the proportion rose to 19% among those whose membranes were ruptured for more than 12 hours. Other significant factors were the year of delivery, whether antiretroviral therapy was used, the mother's AIDS status, her CD4 percentage (an indicator of the severity of her illness), and the child's birth weight and gestational age. Only one factor that the researchers examined--the mode of delivery--was not related to the risk of vertical transmission at the univariate level.
The researchers used logistic regression to calculate odds ratios estimating the strength of the association between the length of time membranes were ruptured and the infant's HIV status, controlling for the mother's CD4 percentage, mode of delivery, receipt of antiretroviral therapy and the infant's birth weight. According to the results, each one-hour increase in duration of ruptured membranes represented a 2% increase in the risk of transmission (odds ratio, 1.02). Further analyses using additional variables produced similar odds ratios.
As in the univariate analysis, the mode of delivery was the only factor that did not contribute to the risk of vertical transmission. The risk was significantly elevated for women with low CD4 percentages (odds ratios, 1.9-2.3) and those whose infants weighed less than 2,500 g at birth (1.8); it was reduced if the woman, her infant or both received antiretroviral therapy (0.3-0.6).
Finally, an examination of interactions between the duration of membrane rupture and the other variables suggested that the probability of vertical transmission was affected by an AIDS diagnosis. For women with AIDS, this probability was 8% if the duration of membrane rupture was two hours, and it rose to 31% if the membranes were ruptured for 24 hours.
According to the researchers, the large number of deliveries included in their analyses allowed them to examine the relationship of duration of membrane rupture to vertical HIV transmission in greater detail than was previously possible. They suggest that future studies incorporate information on maternal viral load and investigate the relative importance of duration of ruptured membranes among HIV-infected women receiving various antiretroviral treatments.--J. Tomarken
1. International Perinatal HIV Group, Duration of ruptured membranes and vertical transmission of HIV-1: a meta-analysis from 15 prospective cohort studies, AIDS, 2001, 15(3):357-368.