What is the association between erythromycin exposure in infancy and infantile hypertrophic pyloric stenosis?

Epidemiological studies suggest erythromycin exposure in infancy is associated with a 2–3-fold increased risk of infantile hypertrophic pyloric stenosis, highest during the first 14 days of life, with an estimated risk of 2.6% (95% CI 1.5–4.2%) versus 0.1–0.2% in the general population.