This is a phase-III multi-center double-blind randomized controlled trial of 8,000 individuals undergoing a scheduled or prelabor cesarean delivery who are randomized to either adjunctive azithromycin prophylaxis or to placebo. Both groups also will receive standard of care preoperative antibiotics (excluding azithromycin). The primary endpoint is a maternal infection composite defined as any one of the following up to 6 weeks postpartum: endometritis, wound infection, abscess, septic thrombosis, sepsis, pneumonia, pyelonephritis and breast infection.