BACKGROUND: Several studies have shown tracking of obesity from childhood to adult life. People who develop obesity in adult life may therefore have had a particular path of growth from birth through childhood. OBJECTIVE: To examine the relationship of obesity to size at birth and childhood growth. DESIGN: Birth cohort study. PARTICIPANTS: A total of 5210 individuals alive and living in Finland in 1997, who were born at the Helsinki University Central Hospital between 1924 and 1933 and who went to school in Helsinki were sent a questionnaire in order to get information about adult weight and height. Detailed birth and school health records were available for all subjects. In all, 3847 responded and 3659 (1552 men and 2107 women) with adequate data are included in the present study. MEASUREMENTS: Incidence of obesity based upon lifetime maximum body mass index (BMI) ascertained from a postal questionnaire and defined as a BMI>30 kg/m2. The main explanatory measurements were size at birth and childhood growth in height, weight and BMI. RESULTS: The cumulative incidence of obesity was 34.2% in men and 33.9% in women. The incidence rose with increasing birth weight and ponderal index (birthweight/length3; P=0.01 and P=0.04, respectively). These associations were statistically significant only among males. By the age of 7 y the mean weights, heights and BMI of people who later became obese exceeded the average and remained above average at all ages from 7 to 15 y. In both men and women there was a 3-fold increase in obesity associated with a BMI>16 kg/m2 at age 7 compared with a BMI<14.5 kg/m2 (P<0.0001). Boys and girls whose mothers had a high BMI in pregnancy had more rapid childhood growth and an increased risk of becoming obese. This effect was stronger among boys (P=0.008). CONCLUSIONS: Obesity is initiated early in life. These results emphasise the importance of early preventive measures for its treatment. International Journal of Obesity (2001) 25, 735-740 | 
