Polycystic ovary syndrome (PCOS) is the most common endocrine disorder in women of reproductive age (prevalence 6-15%). Pregnant women with PCOS have an increased risk of gestational diabetes mellitus (GDM; OR 3.7), preterm birth (PTB; OR: 1.8) and preeclampsia (OR: 3.5). Their newborns are at risk for admission to the NICU (OR: 2.3) and perinatal death (OR: 3.1). So far risk reduction strategies (e.g. metformin) to prevent complications of PCOS pregnancies have not been effective. Myo-inositol is an endogenous insulin-mimetic that promotes intracellular glucose uptake. In (pre-) clinical studies PCOS is associated with intracellular depletion of myo-inositol, leading to insulin and testosterone overproduction. This hormonal dysregulation may lead to pregnancy complications and adverse short- and long-term health outcomes of PCOS offspring. Myo-inositol effectively decreases glucose, insulin and testosterone levels. In 4 previous non-PCOS RCTs among pregnant women myo-inositol supplementations led to a striking 65% decrease of GDM, PTB and PE. This Dutch multicenter study aims to evaluate the effectiveness of myo-inositol versus a placebo among 460 pregnant women with PCOS. Primary endpoint: a composite of GDM, PTB and PE. Secondary endpoints: maternal physical and mental health, fetal and neonatal health and cost-effectiveness. Based on a potential 35% reduction, pregnancy complications will be avoided in 1860 women in the Netherlands each year, total health care savings: €10 million. This study will be conducted by a multidisciplinary team involving PCOS patients, and specialists from the fields of reproductive and perinatal medicine, endocrinology, HTA and statistics. This study is supported by the PCOS foundation; a special focus group represents our target population and is actively involved in all stages of this study. We are confident that this study is a feasible project and will address an important need for improved pregnancy outcomes for our patients.