According to a recent Centers for Disease Control and Prevention investigation of a cluster of cases of hypertrophic pyloric stenosis, this condition seemed to be associated with prophylactic use of erythromycin in children approximately two weeks of age with possible exposure to pertussis. Concern has arisen about the safety of using erythromycin to treat chlamydia and other infections during pregnancy. Because erythromycin can cross the placenta, erythromycin given during pregnancy might also be associated with an increased risk of pyloric stenosis in young infants. Louik and associates looked at data from 1,044 infants with pyloric stenosis who were identified during a case-control surveillance of birth defects in several geographic areas of the United States over 22 years.

Medical information was obtained from the infants' physicians and frequently from the infants' mothers. The authors examined detailed information about all medications used by the mothers from two months before the last menstrual period through the end of pregnancy. Exposure to erythromycin was defined as any reported use of erythromycin during this time. There was no increased risk of infant pyloric stenosis among infants born to mothers who reported using erythromycin during pregnancy.

The authors conclude that although exposure of newborn infants to erythromycin may be a risk factor for the subsequent development of pyloric stenosis, the current study does not demonstrate a similar risk association when erythromycin is taken by the mother early or late in pregnancy.