Objective: This study analyzed mother-to-child HIV transmission

rates by sex and exposure time for babies born to HIV-infected, untreated African women. Methods: Data were analyzed from 2 independent studies done in Malawi during the 1990s. Infections were established by polymerase chain reaction on blood samples. Odds ratios (ORs) for transmission were examined by period at risk: in utero (infected in umbilical cord blood), perinatal (infected in 1st postnatal blood Q4 weeks), and postnatal (later postnatal infection). Results: Among 1394 singleton births, girls were more likely to become infected than boys. For in utero transmission, the OR was 1.4 (95 % CI: 0.9 to 2.2). For transmission during early life (umbilical cord blood not available) the OR was 2.7 (95 % CI: 1.5 to 4.9). However, transmission risks in the perinatal and postnatal infection periods did not differ in boys and girls. Among 303 tested twin-birth pairs, girls were at higher risk than boys for in utero (OR: 2.6; 95% CI: 1.2 to 5.8) and perinatal (OR: 1.9; 95 % CI: 1.0 to 3.7) infection. Recognized mother-to-child transmission risk factors did not explain the higher risk of infection in girls. Conclusions: Girls were at higher risk of early (in utero and peri-natal) HIV infection than boys. It is proposed that minor histo-compatibility reactions between maternal lymphocytes and infant Y chromosomeYderived antigens reduce the risk of HIV transmis-sion in boys