We agree that it is important to establish a standardized international definition of preterm birth. In fact, we concluded our paper by calling for a change in US reporting practices that could facilitate more meaningful international comparisons as well as to more accurately estimate the frequency of preterm birth.

Some definitions as cited by Dr Chabra are as follows: the World Health Organization definition of preterm birth states that “babies born alive before 37 weeks of pregnancy are completed.” The definition of the American Academy of Pediatrics and American College of Obstetricians and Gynecologists of term states that it is “259 days or more or 37 0/7 weeks of gestation.”

The definition of preterm birth per the User Guide for the Natality File published by the Centers for Disease Control and National Vital Statistics states that “births occurring before 37 completed weeks of gestation are considered to be preterm for purposes of classification.”

We point out to Dr Chabra that “less than 37 weeks completed gestation” is equivalent to “less than or equal to 36 completed weeks” in the Materials and Methods section of our report because gestational age is recorded by the completed week in the National Vital Statistics System. Therefore, the group of infants classified as preterm in our analysis is in fact consistent with all the definitions put forth by the World Health Organization, the American Academy of Pediatrics, and the American College of Obstetricians and Gynecologists.