This study will determine if blood levels of anti-HIV drugs in pregnant women change at different stages of pregnancy and if these changes require dosage adjustments in order to maintain adequate drug levels during pregnancy. Anti-HIV medications are recommended for HIV-infected women during pregnancy not only to treat their infection, but also to reduce the chance of passing the virus to the baby during pregnancy. Changes in the body that occur during pregnancy may affect how the body uses and eliminates these drugs, reducing their levels during pregnancy.