Objectives To explore the impact of COVID-19 lockdown on premature birth rates in Denmark Design Nationwide register-based prevalence proportion study. Participants 31,180 live singleton infants born in Denmark between March 12, and April 14, from 2015 to 2020 Main outcome measures The Main outcome measure was the odds ratio of premature birth, per preterm category, during the lockdown period compared with the calendar match period in the five previous years. Results A total of 31 180 newborns were included in the study period, of these 58 were born extremely premature (gestational age below 28 weeks). The distribution of gestational ages was significantly different (p = 0.004) during the lockdown period compared to the previous five years. The extremely premature birth rate during the lockdown was significantly lower than the corresponding mean rate for the same dates in the previous years (odds ratio 0.09 [95 % CI 0.01 - 0.04], p < 0.001). No significant difference between the lockdown and previous years was found for other gestational age categories. Conclusions The birth rate of extremely premature infants decreased significantly (~90 % reduction) during the Danish nationwide lockdown from a stable rate in the preceding five years. The reasons for this decrease are unclear. Identification of possible causal mechanisms might stimulate changes in clinical practice. Ideally, some cases of extreme prematurity are preventable which may decrease infant morbidity and mortality.