Current guidelines use body mass index (BMI) to predict which women have increased risk of developing pregnancy complications and require additional care. BMI is an inaccurate measure to predict risk and leads to many women receiving inappropriate care, makes health outcomes worse and wastes NHS resources. Being overweight (BMI between 25 and 29) or obese (BMI 30 or above) is associated with pregnancy complications including diabetes, preterm birth, and death of mother or baby. Guidelines recommend that obese women receive costly high-risk care including additional referrals, tests and closer monitoring. This is unnecessary for half of obese women who don’t develop pregnancy complications, approximately 87,000 women/year in England. Almost half of overweight women do develop complications, approximately 103,000 women/year. Using obese BMI in guidelines wastes resources and increases anxiety for obese women inaccurately labelled as high-risk, and overlooks many overweight women who do require additional care. BMI does not assess where body fat is stored. When fat is stored in the upper-body there is an increased risk of developing certain diseases. Someone with a “healthy” BMI who stores fat around their waist can have a higher risk than someone with obesity who stores fat on their hips. Alternative measures can identify where body fat is stored (e.g. waist size, ultrasound examination). These are called adiposity measures. This research will look at whether adiposity measures can predict which women will develop complications in pregnancy to help the NHS target care to women who really need it.