5. Dr. Mia Doron, associate professor of pediatrics at University of North Carolina and coauthor of Preemies: The Essential Guide for Babies, says that the majority of premature children will catch up in size within the first year, with only a few who will never catch up. While in some cases there is an obvious medical cause for the discrepancy, in others children it is impossible to tell why they grow smaller. Genetics is one possible cause.