BACKGROUND: Treatment for gestational diabetes mellitus (GDM) aims to reduce maternal hyperglycaemia. The TARGET Trial assessed whether tighter compared with less tight glycaemic control reduced maternal and perinatal morbidity. METHODS AND FINDINGS: In this stepped-wedge, cluster-randomised trial, identification number ACTRN12615000282583, 10 hospitals in New Zealand were randomised to 1 of 5 implementation dates. The trial was registered before the first participant was enrolled. All hospitals initially used less tight targets (fasting plasma glucose (FPG)