PROJECT SUMMARY/ABSTRACT We propose a large multi-center non-inferiority follow-up cohort study designed to evaluate long-term obesity/growth and neurodevelopment after perinatal exposure to adjunctive azithromycin as compared with placebo (plus standard cephalosporin) to prevent surgical site infections at cesarean delivery. The parent C/SOAP randomized trial of adjunctive azithromycin (4/2011–11/2014) demonstrated a significant reduction in maternal infections (endometritis and wound infection) without any significant effect on short-term neonatal outcomes in the azithromycin group. This reduction in maternal infection (a major cause of death) was so significant that azithromycin use was found to be cost-saving – saving ~$360/unscheduled cesarean and $270 million/year in US healthcare costs. However, enthusiasm for adjunctive azithromycin has been tempered given reports of changes induced by azithromycin on the newborn gut microbiome that may be associated with childhood weight gain/obesity and neurodevelopmental impairment. In September 2018, ACOG suggested “consideration” of adjunctive azithromycin, but emphasized the need for studies of its effect on long-term childhood outcomes. In response to these concerns, we proposed an R01 study evaluating pulmonary and gastrointestinal (GI) health after perinatal azithromycin exposure (6th percentile score). Reviewers emphasized the need to evaluate obesity/growth and neurodevelopment (NIH officers advised a separate application due to cap limitations). Thus, in this proposal, we hypothesize that perinatal exposure to adjunctive azithromycin compared to standard cesarean prophylaxis alone is not associated with increased risk of obesity/weight gain or neurodevelopmental impairment (i.e. is non-inferior). We propose a follow-up study of the offspring of the C/SOAP trial cohort at target age of 7 years (range 6-9) at the 14 sites in the parent trial. We expect at least 1410 of 2013 children (70% follow-up) whose mothers were enrolled in C/SOAP will undergo a single study visit incorporating growth/biometry and neurodevelopment assessments to address the following specific aims: 1) To compare the incidence of obese/overweight children (primary outcome), and other biophysical measurements, between cohort offspring aged 7 (range 6-9 years) after perinatal exposure to adjunctive azithromycin versus placebo (standard prophylaxis alone) 2) To compare neurodevelopmental outcomes including cognitive ability (primary outcome), functional and behavioral impairment, and motor proficiency, between cohorts at age 7 (range 6-9 years) after perinatal exposure to adjunctive azithromycin versus placebo This study, along with the companion study evaluating pulmonary and GI outcomes (award notice pending), is the only opportunity to efficiently provide comprehensive long-term safety data from a large US trial to inform and strengthen the newly recommended use of adjunctive azithromycin for cesarean delivery.