Evaluation of Antimicrobial Prophylaxis to Prevent Syphilis in Pregnancy in Patients at Risk in Rio de Janeiro, Brazil

Inclusion Criteria: 1. Uncomplicated, viable pregnancy 2. Elevated risk for syphilis acquisition (one or more of the following): 1. Prior history of STIs last 3 years 2. HIV infection 3. Age \< 21 years 4. In a sexual partnership of \< 3 mo., or \> 3 sexual partners in the last 6 mo. 5. Late initiation of prenatal care (\> 14 weeks of pregnancy) 6. Residence in area where syphilis prevalence is 10% or higher. 3. Ability to provide written informed consent 4. No allergy to penicillin 5. Prenatal care at one of the sites participating in the study 6. Ongoing sexual activity during study period. 7. Negative rapid treponemal test at baseline. Exclusion Criteria: 1. Non-viable pregnancy 2. Very high risk pregnancy 3. Inability to provide written informed consent 4. Allergy to penicillin 5. Positive rapid treponemal test at baseline 6. Lack of sexual activity during study period. 7. Any persisting coagulation disorder that would contraindicate IM injections.