This retrospective study aims to compare discharge criteria and their impact on the length of stay using two neonatal intensive care units from two countries. The possible discharge criteria include temperature control, apnea observation, periodic respiration, least weight limit, and parents' readiness.

The data were collected retrospectively in each study site.

Aims: To compare discharge criteria and their impact on the length of stay among preterm infants in Japan and Finland. The possible discharge criteria include temperature control, apnea observation, periodic respiration, least weight limit, and parents' readiness.

Study design and settings: This is a retrospective comparison study in two NICUs: a Level IV in Japan and a Level III in Finland.

Patients: Eligible infants are those who were born between 28+0 and 31+6 weeks of gestation and who were discharged home from the study sites between January 2020 and December 2021. Infants will be excluded if (1) they have major anomalies at birth, (2) they are not discharged home from the study sites, or (3) they are discharged home with oxygen and/or after tracheostomy.

Outcome measures: The primary outcomes are the most common last discharge criterion between the two NICUs, and the potential shortening effects of each discharge criterion on the length of stay. The secondary outcome is the postmenstrual age (PMA) for each discharge criterion.

Statistics: The comparison between two continuous variables will be carried out by the student t test.

Ethics: The Ethics Committee of Nagano Children's Hospital gave ethical approval to this study. The study permission was obtained in both study sites.

Inclusion Criteria:

• Born between 28 and 31 weeks of gestation and
• Discharged home from the study sites between January 2020 and December 2021

Exclusion Criteria:

• Having major anomalies at birth
• Need for home oxygen therapy, tracheostomy, and/or gastrostomy at discharge

Newborns who were born and taken care of in two neonatal intensive care units (NICUs): a Level IV NICU at Nagano Children's Hospital in Japan and a Level III NICU at Turku University Hospital in Finland.

It might be possible only after the necessary procedures have been followed including study permissions in both study sites.