This is a retrospective single-center cohort study. The study setting is a level IV neonatal intensive care unit (NICU) at Nagano Children's Hospital in Nagano, Japan. We will retrospectively collect the data of those who were admitted to Nagano Children's Hospital NICU between January 1, 2015, and December 31, 2019. Eligible infants will be identified using our neonatal database and clinical records. The extracted data includes maternal background information and clinical course, infant's status at birth, the clinical course in NICU, and the follow-up data at 6 and 18 months of corrected age.~Among the eligible infants, those with a z score of birth head circumference < -2 will be classified into the Microcephaly group and others into the Control group. The z score is calculated using the Japanese growth standard.
This is a retrospective single-center cohort study. The comparison in short- and long-term outcomes will be made between those with and without primary microcephaly in infants weighing ≤ 500 g.