Over the last 30 years, the management of gestational diabetes mellitus (GDM) has evolved from an almost randomised controlled trial (RCT) “evidence-free” condition, to a much studied, and discussed, clinical entity with varied guidelines around the world, focusing on GDM at 24-28 weeks gestation. However, GDM diagnosed early in pregnancy (̴30-70% of all GDM) is associated with worse outcomes than pregnancies with GDM developing later in pregnancy. The Treatment Of BOoking Gestational diabetes Mellitus (TOBOGM) study is the only international RCT of treatment of GDM in early pregnancy that involving masked controls. Participating women had diabetes risk factors and an oral glucose tolerance test (OGTT) before 20 weeks’ gestation. Those with GDM by WHO-2013 criteria (n=802) were randomised to either treatment or a repeat OGTT at 24-28 weeks gestation. Nested studies allowed comparison of pregnancy outcomes among women with GDM with higher and lower glucose values, and before and after 14 weeks gestation. Women without early GDM form an epidemiological cohort. The TOBOGM results clearly showed that early treatment reduced a primary outcome composite of important neonatal complications; babies had less fat and days in the special care unit were down 0.8 days. Quality of life and breastfeeding were significantly improved. Early testing and treatment of those with early GDM was A$5500 cost saving if before 14 weeks’ gestation. However, TOBOGM also showed that caution is needed in selecting thresholds for early treatment, with a 75% increased risk of small for gestational age babies, with the potential for long term consequences. This evidence has been used in the latest ADIPS-25 recommendations to raise the threshold for diagnosing GDM and promote more early pregnancy testing. It is now clear that GDM represents pre-existing metabolic abnormalities with implications that GDM should now be seen as a lifecourse, not solely a pregnancy-related condition.