Preterm birth refers to a liveborn infant delivered prior to 37 weeks’ gestation. Preterm birth is the leading cause of neonatal mortality in the world. Lifelong morbidities continue to affect many preterm babies who survive. In this presentation, Dr Tielsch explains methods used for estimating gestational age. Although calculations based on the last menstrual period is the most commonly used, foetal anthropometry within the first trimester through an ultrasound examination provides the most accurate estimate. In 2010, the incidence of preterm births was about 11% of all life births globally. Dr Tielsch also presents a new paradigm for defining and classifying preterm births proposed by the Global Alliance for the Prevention of Preterm and Stillbirths (GAPPS) working group.