http://www.cdc.gov/hiv/pubs/faq/faq14.htm
Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
This frequently asked question page provides information as to why the Centers for Disease Control and Prevention (CDC) recommends human immunodeficiency virus (HIV) screening for all pregnant women. HIV testing during pregnancy is important because antiviral therapy can improve the mother’s health and greatly lower the chance of an HIV-infected pregnant woman passing HIV to her infant before, during, or after birth. The treatment is most effective when started as early as possible during pregnancy. CDC recommends HIV screening for all pregnant women because risk-based testing approaches miss many women who are infected with HIV.