Supplemental Table 1.

Outcome definitions and data sources

Data sources or outcome	Norway	Sweden	Denmark
Data sources	Medical Birth Registry of Norway	Swedish Pregnancy Register	Danish National Patient Register The Danish Civil Registration System Danish Medical Birth Register
Preterm birth	Live birth or stillbirth <259 da	1) ICD-10 O60.1 (Spontaneous preterm labor with preterm delivery) or 2) live birth or stillbirth <259 da	Live birth or stillbirth <259 da
Medically-indicated preterm birth	Live birth or stillbirth <259 d with induced labor or cesarean delivery without labor	1) ICD-10 O60.3 (Preterm birth without spontaneous start of labor) or 2) live birth or stillbirth <259 d with induced labor or cesarean delivery without labor	Live birth or stillbirth <259 d with induced labor, or cesarean delivery without labor. If there is a code indicating rupture of membranes without regular contractions, the birth is reclassified as spontaneous preterm.
Spontaneous preterm birth	Live birth or stillbirth <259 d with spontaneous start of labor	1) ICD-10 O60.1 (Spontaneous preterm labor with preterm delivery) or 2) live birth or stillbirth <259 d with spontaneous start	Live birth or stillbirth <259 d that is not classified as induced (provided above) and is not unclassifiable
Very preterm birth	Live birth or stillbirth <223 d	Live birth or stillbirth <223 d	Live birth or stillbirth <223 d

ICD-10, International Classification of Diseases, Tenth Revision.

Oakley et al. Preterm birth and COVID-19 mitigation measures in Scandinavia. Am J Obstet Gynecol 2022.

^aGestational age is based on routine ultrasound measurements when this is available (approximately 98% of births); otherwise, the last menstrual period is used.