Gestational diabetes (GDM) is a significant health problem affecting one in every 12 pregnant women or over 5200 women in New Zealand annually. GDM has a major, negative impact on maternal and perinatal health with lifelong consequences. It is unclear what intensity of glycaemic control during GDM treatment is best for mother and infant. The TARGET randomised trial will evaluate the implementation of tighter treatment targets for blood sugar control compared with current less stringent targets in women with GDM. We will compare the risk of the infant being born large for gestational age, which is strongly associated with significant early life and later health problems. Other relevant outcomes will be assessed. This unique trial will allow for the sequential implementation of the new, nationally recommended tighter treatment targets for women with GDM and, for the first time, establish if there are true benefits, without harm, to tighter treatment targets.