This study quantified the opportunities for reducing perinatal HIV transmission if pregnancy status was available on laboratory reports. For mothers whose HIV status was known before pregnancy and who had an HIV test during pregnancy, the highest transmission rate occurred in those without a viral load test (7.0%). This analysis suggests pregnancy status on laboratory reports would be useful for targeting women who have an HIV test during pregnancy but no viral load test. Health Departments should continue to work on the identification of pregnancy status on HIV laboratory reports and should require this information in their laboratory reporting guidelines.