ObjectiveTo evaluate the effectiveness and cost-effectiveness of the National PReCePT Programme (NPP) in increasing use of magnesium sulfate (MgSO4) in preterm births.DesignBefore-and-after study.SettingMaternity units (N=137) within NHS England and the Academic Health Science Network (AHSN) in 2018.ParticipantsBabies born ≤30 weeks’ gestation admitted to neonatal units in England.InterventionsThe NPP was a quality improvement (QI) intervention including the PReCePT (Preventing Cerebral Palsy in Pre Term labour) QI toolkit and materials (preterm labour proforma, staff training presentations, parent leaflet, posters for the unit and learning log), regional AHSN-level support, and up to 90 hours funded backfill for a midwife ‘champion’ to lead implementation.Main outcome measuresMgSO4uptake post implementation was compared with pre-NPP implementation uptake. Implementation and lifetime costs were estimated.ResultsCompared with pre-implementation estimates, the average MgSO4uptake for babies born ≤30 weeks’ gestation, in 137 maternity units in England, increased by 6.3 percentage points (95% CI 2.6 to 10.0 percentage points) to 83.1% post implementation, accounting for unit size, maternal, baby and maternity unit factors, time trends, and AHSN. Further adjustment for early/late initiation of NPP activities increased the estimate to 9.5 percentage points (95% CI 4.3 to 14.7 percentage points). From a societal and lifetime perspective, the health gains and cost savings associated with the NPP effectiveness generated a net monetary benefit of £866 per preterm baby and the probability of the NPP being cost-effective was greater than 95%.ConclusionThis national QI programme was effective and cost-effective. National programmes delivered via coordinated regional clinical networks can accelerate uptake of evidence-based therapies in perinatal care.