Obesity during pregnancy increases the risk of development of gestational diabetes (GDM), which can develop into type 2 diabetes postnatally. The UK has guidelines on diet and exercise in place following a positive result for GDM, however it is unclear how results from a GDM test may affect behaviours and weight gain in women with obesity during pregnancy. This secondary analysis of a randomised control trial; the UK pregnancies better eating and activity trial (UPBEAT), aimed to assess how a negative or positive diagnosis of GDM affects the diet, physical activity levels and weight gain of pregnant women with obesity. 1031 pregnant women who were positively and negatively diagnosed with GDM were randomly split into two groups for eight weeks. The intervention group received additional appointments with a health coach as well as regular antenatal appointments. The results showed that women who received a negative result to GDM gained significantly more weight than those with a positive diagnosis regardless of which treatment group they were allocated. GDM status also affected dietary intake, however no changes in amount of physical activity were seen with either diagnosis. It was concluded that pregnant women with obesity gained more weight if they received a negative result to GDM and a positive GDM result is associated with lower weight gain and dietary changes. This highlights the need for additional diet and weight gain strategies for pregnant women with obesity who return a negative GDM result. Clinicians could use this study to stress the importance of healthy eating and exercise to pregnant women with obesity, even if a negative GDM test is received.