Preterm birth is the leading cause of child mortality globally, with many survivors experiencing long-term adverse consequences.

Preliminary evidence suggests that numbers of preterm births greatly reduced following implementation of policy measures aimed at mitigating the effects of the COVID-19 pandemic.

We aimed to study the impact of the COVID-19 mitigation measures implemented in the Netherlands in a stepwise fashion on March 9, March 15, and March 23, 2020, on the incidence of preterm birth.

We used a national quasi-experimental difference-in-regression-discontinuity approach.

We used data from the neonatal dried blood spot screening programme (2010–20) cross-validated against national perinatal registry data.

Stratified analyses were done according to gestational age subgroups, and sensitivity analyses were done to assess robustness of the findings.

We explored potential effect modification by neighbourhood socioeconomic status, sex, and small-for-gestational-age status.