Gestational diabetes (GDM) is a significant health problem affecting one in every 12 pregnant women or over 5,200 women in New Zealand annually. GDM has a major negative impact on maternal and perinatal health with lifelong consequences. There is no consensus as to the degree of high blood glucose needed for the diagnosis of GDM or when treatment will be beneficial, due to a lack of high-quality evidence. Our unique, randomised trial compares important health outcomes for mothers and babies of treating women with GDM by the current criteria used in New Zealand, with newly proposed criteria that use a lower threshold and will diagnose more women as having GDM. Our results will show which diagnostic criteria is best for the health of mothers and babies, which is more cost-effective, and so provide the necessary information to guide clinical practice and policy in New Zealand, with global relevance.