Depending on where you live, local organizations may be offering free testing today. Of course, depending on where you live, that may be true every day. Check with your local health department, Planned Parenthood, etc for details.
HIV testing can help to stop the spread of the disease. One would hope that adults would do what they could to avoid spreading the virus if they knew they were positive, but testing of pregnant women, in particular, can significantly decrease the risk of vertical transmission (mother to baby).
A woman who takes antiretroviral drugs while pregnant significantly decreases the probability of transmission to her baby. Prior to the use of antiretrovirals in pregnancy, the transmission rate was nearly 30% for term babies and much higher for preemies. Now, with antepartum treatment of the mom, followed by six weeks of antiretroviral therapy for the baby, the transmission rate in term infants is 2-7%. This can be reduced to about 1% with elective C-sections prior to labor. Even when the mom doesn't know she's HIV positive until she presents in labor (women with no or very limited prenatal care), transmission rates can be decreased by treating the baby for 6 weeks after birth.