Optimizing PreTerm Infant Ampicillin Dosing

The goal of this clinical trial is to learn if preterm infants who are prescribed antibiotics shortly after birth can safety receive a shorter course of antibiotics (24 to 36 hours instead of 48 hours). The main questions it aims to answer are: * Does short-course ampicillin provide high enough levels of ampicillin at 48 hours? * Is short-course ampicillin safe for preterm infants to receive? Preterm infants who are being prescribed ampicillin by their doctor and enroll in the study will stop ampicillin after a shorter than typical course, and researchers will collect blood samples to measure their ampicillin levels and follow them clinically to see how they do after receiving short-course ampicillin. Participants will: * stop ampicillin earlier than 48 hours (between 24 to 36 hours, depending on how premature they are and the dosing of ampicillin their doctor has prescribed) * have one or more blood samples collected, including one around 48 hours from when they started ampicillin * have their data collected until 30 days after they receive short-course ampicillin, or until hospital discharge, whichever is sooner