This project will evaluate a new way to improve the health and wellbeing of women with gestational diabetes (GDM) and their babies. It uses a digital health app called GDm-Health which helps mothers and their health care professionals manage blood glucose control, to deliver a life-style intervention targeted at improving physical activity levels. GDM is a condition in which a woman develops high blood glucose levels during pregnancy, increasing the risk of complications for the mother and baby. Poor control of the condition increases the risk of babies that are too large, have low blood glucose and are jaundiced. Mothers are at increased risk of pre-eclampsia and caesarean section. Long term, mothers and children are at higher risk of being overweight and developing type 2 diabetes. It is known that increasing physical activity in mothers is an effective way to bring blood glucose levels closer to normal. This has real benefits in reducing complications during pregnancy and problems during birth caused by larger-than-average babies. It also helps mothers to lose weight quickly after birth, and reduces the risk that they go on to develop type 2 diabetes. Unfortunately, mothers often feel isolated, tire easily, and lack the motivation to maintain levels of physical activity during pregnancy. It is therefore very important that mothers with gestational diabetes receive continuous support and encouragement. We have developed a programme; 'Stay Active', for women with GDM that includes several behavioural change techniques such as motivational interviews and goal-setting, and provides follow-up contacts to help mothers improve or maintain their physical activity levels during pregnancy. In this project we will test if an updated version of GDm-Health can effectively deliver the Stay Active programme to mothers with GDM. The resulting GDm-Health Stay Active system will reinforce behaviour change using goal setting, performance feedback, monitoring of activity levels and encouragement from the healthcare team. GDm-Health Stay Active will also provide an easily understood resource of NHS-approved information and videos on how to carry out simple exercises. The study will follow PA levels, control of blood glucose levels, and outcomes for mothers and babies together with patient satisfaction with care, app usage and overall costs of treatment. It will provide evidence on the clinical efficacy and healthcare costs of GDm-Health Stay Active in a group of patients for whom we know that physical activity is beneficial.