INS and obesity due to melanocortin 4 receptor deficiency: The Efficacy of Metformin in Pregnant Obese Women, a Randomised controlled (EMPOWaR) trial is designed to establish whether improving insulin sensitivity in pregnancy mitigates some of the adverse risk associated with obesity, with the primary aim of examining the effect on the birth weight of the baby, using birth weight as a surrogate marker of future life risk of obesity.