2.2. Experience of serious illness in early life

Information on illnesses and injuries during early life, including hospital admission, age at admission, diagnosis, name of hospital, and length of hospital stay was obtained from mothers' reports across childhood and study participants' reports from age 18 up to 25 years. Any reported admissions were checked with the hospitals concerned and coded according to the International Classification of Diseases, Revision 8 (1965). We defined serious illness as any experience of physical illness before the age of 25 years that required hospital admission of at least 28 days (yes/no).^20,21 Additional variables identified are included in the following: (1) the total number of periods during early life (ie, infancy [0-4 years], childhood [5-14 years], adolescence [15-19 years], or early adulthood [20-24 years]) that serious illness was experienced (range: 0 [no serious illness reported] to 4 [serious illness reported in all 4 periods of early life]); and (2) distinguishing potentially disabling illnesses (ie, polio [ICD8 codes: 040-046], diseases of central nervous system [320-349], mental retardation [310-315], circulatory diseases [400-458], musculoskeletal [710-738], congenital anomalies [740-759], and accidents [800, 810, and 820-860]) from other illnesses.²⁰