Babies born between 37 and 42 weeks of gestation are considered to have been born at the optimal time, and earlier or later delivery is known to be associated with increased risk of a range of outcomes from neonatal complications to impaired outcomes in terms of physical and intellectual ability and even death. Originally, there was an uncertainty about the length of gestation, because the basis for calculating gestational age (from the first day of the last menstrual period) was acknowledged as inaccurate due to conception at different points during the menstrual cycle and variation in an individual woman's cycle, thus a wide definition of full term was developed. With demand-led timing of delivery increasing around the world, many more babies are therefore delivered electively at 37–38 completed weeks of gestation, despite guidance from a range of official bodies.1

This rather artificial concept is further cemented in place with the use of ‘full term’ comparison groups in many studies as the reference population. Indeed there is currently much research interest in the moderate or late …