evidence: Prospectively collected data were derived from the Netherlands Perinatal Registry (PRN).16 PRN is a Dutch nationwide database that contains demographics and information about mothers and newborns, courses and outcomes of pregnancy, and content and organisation of care, all entered by healthcare providers. Around 96% of all deliveries from 20 weeks of gestation onwards are registered in PRN. The database consists of three linked and validated registries: the national obstetric database for midwives (LVR-1), the national obstetric database for gynaecologists (LVR-2) and the national neonatal/pediatric database (LNR).17 ,18 The study period was from January 2000 to December 2007. We chose this period to avoid bias, because medicinal termination of pregnancy was not commonly used at that time, the majority of pregnancy terminations being performed surgically. All multiple births and births of infants with a congenital anomaly in index pregnancies were excluded. Also, all women where labour was induced or a planned caesarean section was performed before 37 weeks’ gestation, that is, iatrogenic preterm deliveries, were excluded (figure 1).