The flow diagram of development of the 2007 to 2011 birth cohort.

NHSIC, the National Health Screening Program for Infants and Children; P07.0, the code of BW <1000 g by the Korean Classification of Diseases; P07.1, the code of 1000 g ≤ BW <2500 g in the Korean Classification of Diseases; NHIS, the National Health Insurance Service.