OBJECTIVE: To assess the association between prenatal antibiotics, including 
erythromycin, and infantile hypertrophic pyloric stenosis in a large cohort of 
infants.
METHODS: This was a retrospective cohort study of births to women enrolled in 
Tennessee Medicaid/TennCare, 1985-1997. Prescriptions for erythromycin, 
nonerythromycin macrolides, and other antibiotics were identified from pharmacy 
files linked with birth certificate files. The primary study outcome was 
development of pyloric stenosis in the infant, identified from linked hospital 
discharge diagnosis and surgical procedure codes.
RESULTS: The cohort included 260,799 mother/infant pairs. Among these women, 
13,146 filled prescriptions for erythromycin (50.4 per 1000), and 621 filled 
prescriptions for nonerythromycin macrolides (2.4 per 1000). There was no 
association with prenatal erythromycin prescription and infantile hypertrophic 
pyloric stenosis either after 32 weeks' gestation (adjusted odds ratio 1.17, 95% 
confidence interval, 0.84, 1.64, P =.33) or at any time during pregnancy 
(adjusted odds ratio 1.15, 95% confidence interval 0.84, 1.56, P =.36). There 
was an association between maternal prescriptions for nonerythromycin macrolides 
and infantile hypertrophic pyloric stenosis (adjusted odds ratio 2.77, 95% 
confidence interval 1.22, 6.30, P =.01).
CONCLUSION: The hypothesized association between erythromycin and infantile 
pyloric stenosis was not seen. Causal inference from the association between 
prenatal nonerythromycin macrolides and infantile hypertrophic pyloric stenosis 
is limited by the small number of affected children and the evidence of other 
differences between users of nonerythromycin macrolides and controls.