This thesis is based on the following four papers: Paper I MG Eide, N Øyen, R Skjærven, ST Nilsen, T Bjerkedal, GS Tell. Size at birth and gestational age as predictors of adult height and weight. Epidemiology; 2005;16(2):175-181. Paper II MG Eide, N Øyen, R Skjærven, T Bjerkedal. Association of birth size, gestational age, and adult height with intelligence: evidence from a cohort of 317 761 Norwegian men. (Under review) Paper III MG Eide, N Øyen, R Skjærven, LM Irgens, T Bjerkedal, ST Nilsen. Breech Delivery and Intelligence: A Population-Based Study of 8,738 Breech Infants. Obstet Gynecol 2005;105(1):4-11. Paper IV MG Eide, R Skjærven, LM Irgens, T Bjerkedal, N Øyen. Associations of birth defects with adult intellectual performance, disability and mortality: population-based cohort study. (Under review)