Preterm birth, defined as birth before 37 completed weeks of gestation, is one of the most important problems in medicine, affecting at least 15 million births throughout the world in 20161 and almost 1 in 10 births in the United States in 2015.2 Preterm birth is the leading cause of perinatal death among infants who are born without major congenital anomalies and is a major cause of both short- and long-term morbidity and chronic health problems.3,4 Accordingly, the societal costs are substantial. For example, the societal cost of prematurity in the United States was estimated to be $26.2 billion per year in 20065; the cost is assuredly higher in today’s dollars.