The thymus gland is a specialized organ in the chest that plays a central role in the adaptive immune system throughout development until puberty. In response to stress, the fetal thymus gland may shrink, or involute. The investigators propose a prospective cohort study that will enroll pregnant women admitted to labor and delivery for the management of preterm labor and/or preterm premature rupture of membranes from 28-36 weeks gestation. Based on sonographic thymus measurements, the investigators will develop a clinical prediction tool to identify babies who are at increased risk for adverse neonatal outcomes. A reliable non-invasive predictor of adverse neonatal outcome using thymic ultrasound measurements has the potential to affect clinical management, improve outcomes for premature babies, and direct further research efforts.