HIV transmission to the fetus from mother occurs most commonly at what gestational age?

Virologic analysis of aborted fetuses indicates that HIV can be transmitted to the fetus as early as the first and second trimester of pregnancy. However, maternal transmission to the fetus occurs most commonly in the perinatal period. Studies indicate that the relative proportions of mother-to-child transmissions were 23-30% before birth, 50-65% during birth, and 12-20% via breast-feeding.