Risk factors associated with preterm birth in singletons born after IVF: a national cohort study.

Research Question: What are the risk factors for prematurity other than intrauterine growth restriction in singletons after IVF?

Design: Data were collected from a national registry, based on an observational prospective cohort of 30,737 live births after assisted reproductive technology (fresh embryo transfers: n = 20,932 and frozen embryo transfer [FET] n = 9805) between 2014 and 2015. A population of not-small for gestational age singletons conceived after fresh embryo transfers and FET, and their parents, was selected. Data on a number of variables were collected, including type of infertility, number of oocytes retrieved and vanishing twins.