To assess the benefits and risks of progesterone therapy for women at increased risk of spontaneous preterm birth (SPB) and to make recommendations for the use of progesterone to reduce the risk of SPB and improve postnatal outcomes. To administer or withhold progesterone therapy for women deemed to be at high risk of SPB. Preterm birth, neonatal morbidity and mortality, and postnatal outcomes including neurodevelopmental outcomes. Intended Users Maternity care providers, including midwives, family physicians, and obstetricians.