The risk of human immunodeficiency virus-1 infection in twin pairs born to infected mothers in Africa.

We examined birth order and delivery route as risk factors for mother-to-child transmission of human immunodeficiency virus (HIV)-1 in 315 twin pairs born in Malawi during 1994-1998. No antiretroviral drugs were administered to these subjects. Infections were detected by polymerase chain reaction and were stratified as having occurred either in utero, perinatally, or postnatally. Risk of in utero infection for 630 infants (39 infections) did not differ by birth order (first born, 6.3%; second born, 6.0%). Similarly, in 260 vaginally delivered infants evaluated for perinatal infection (45 infections), risk did not differ by birth order (first born, 15.9%; second born, 18.7%); risk of perinatal infection was significantly lower in cesarean-delivered infants (odds ratio, 0.19 [95% confidence interval, 0.02-0.78]). There was no effect on postnatal transmission rates. Thus, in contrast to the authors of earlier studies, we did not find birth order to be an important risk factor for infection in twins. These findings indicate that birth-canal exposure is not a major contributor to a newborn's risk of HIV-1 infection.