Preterm infants who developed NEC had significantly more antibiotic exposure prior to developing NEC than infants who did not develop NEC. In the 27–30 week group, while the infants demonstrated the same relationship between antibiotic exposure and NEC, the odds ratio for the association between number of antibiotic exposures and NEC for the 27–30 week group was less than half when compared to the 23–26 week group. This difference may be due to the fact that infants who are of lower gestational age and birth weight are at higher risk for infection than infants who weigh more than 1500 g.46 However, if maternal infections alter the immune system of the fetus, this expression may only be seen in the earliest periods after birth when the infants’ systems are more immature, and thus more susceptible to infection.