Objective:To estimate the prevalence ofin uterotransmission of HIV‐1 through the second trimester.Material and methods:One hundred consecutive, unselected, intact fetuses, beyond 15 weeks gestational age (mean, 22.4 weeks) were studied. These were obtained following spontaneous intrauterine deaths (n = 4), miscarriages (n = 4), and elective mid‐trimester terminations (n = 92), eight of which were fetuses with malformations from HIV‐1‐positive pregnancies. Coded DNA extracts from the fetal thymuses were tested blindly by polymerase chain reaction in three laboratories using a total of six different primer pairs.Results:Two thymuses tested positive [95% confidence interval (Cl), 0.2‐7]. Results from the three laboratories were consistent in all 100 cases. The two fetuses with HIV in the thymus both tested positive in other organs, demonstrating systemic HIV infection. The first fetus, whose mother had advanced AIDS, had diedin uteroand had diffuse toxoplasmosis. The second died following extremely premature delivery in a pregnancy complicated by repeated bleeding. HIV infection was observed in none of the 92 fetuses that resulted from elective mid‐trimester terminations (95% Cl, 0‐4).Conclusion:The frequency of earlyin uteroHIV infection appears to be low, compared with transmission rates in infants born to HIV‐1‐infected mothers, suggesting that transmission occurs mostly later in pregnancy and/or at delivery. Specific risk factors may have implications in the occurrence of early as opposed to late transmission.AIDS 1995,9:359‐366