This study is part of the Helsinki Birth Cohort Study (HBCS) which in the epidemiological part included 8760 men and women born at Helsinki University Central Hospital. They were born between 1934 and 1944, attended child welfare clinics in the city and were living in Finland in 1971, when a unique identification number was assigned to all residents of Finland. The majority of the children (77%) also went to school in Helsinki. The birth records included data on weight and length at birth and gestational age. Records from child welfare clinics and school health care include serial measurements of weight and height. On average, the participants had 11 measurements between birth and two years, and 9 measurements between 2 and 11 years. Details of the birth, child welfare and school health records have been described in detail elsewhere 
Of the original cohort, 2003 randomly selected individuals participated in a detailed clinical examination between the years 2001–2004. The clinical study protocol was approved by the Ethics Committee of Epidemiology and Public Health of the Hospital District of Helsinki and Uusimaa. Written informed consent was obtained from each participant before any study procedure was initiated.
All measurements were taken by a team of trained research nurses. Height was measured in light indoor clothing without shoes to the nearest 0.1 cm and weight to the nearest 0.1 kg. Body mass index (BMI) was calculated as weight in kilograms divided by the square of height in meters. Waist circumference was measured midway between the lowest rib and the iliac crest. Lean body mass and body fat mass were measured by bioelectrical impedance analysis (BIA) using InBody 3.0 eight-polar tactile electrode system (Biospace Co. Ltd, Seoul, Korea) 
Using the occupations recorded on the birth, child welfare clinic and school health care records, we grouped fathers into three socioeconomic groups (manual workers, lower-middle and upper-middle classes), originally based on a nine-group social classification system by Statistics Finland. We used the highest group obtained from these three sets of records. Social class in adulthood, based on subjects' own occupation, was derived from Census data in 1980. Physical activity and smoking habits were obtained from a self-administered questionnaire. Being physically active was defined as taking at least moderate exercise three or more times per week. Subjects were defined as smokers if they smoked one or more cigarettes per day.
In the present study, we restricted our analyses to the 606 individuals who took part in the UKK 2-kilometer walk test, in order to examine the effects of fetal and childhood growth on cardiovascular performance among normally active men and women in later life. The UKK 2-km walk test is an indirect measurement of CRF, which can be used within a majority of the adult population without requirements of maximal physical efforts. It has been validated against maximal effort test by treadmill or bicycle ergometry in multiple populations including the obese and elderly 
During the test, subjects are directed to walk through a 2-km course on the flat ground as rapidly as possible. The test results are expressed as fitness index, or alternatively as VO2max
, by using a formula in which subject's age, sex, BMI, time spent in walking and heart rate are taken in to account.
Several health status requirements for the UKK walk test were set. A subject was excluded if he/she had any of the following conditions affecting the walk test: myocardial infarction within the past year, unstable blood pressure, CHD with symptoms, active arthritis or other joint disorder with pain, arrhythmia, asthma or other breathing difficulties, and if the subject had other medical restrictions concerning physical activity. Subjects on drug therapy affecting pulse rate were also excluded from the analyses. Individuals (n
25) whose test result indicated a VO2max
status under 10 ml/kg/min were excluded from further analysis because those with the VO2max
value of this low can not reach the intensity required in the UKK- test, and thus 581 individuals remained in the study.
A multiple linear regression was used to assess the associations of birth size, childhood height, weight and BMI with CRF. The basic models were adjusted for age and in the pooled analysis also for sex. Further analysis included the adjustments for gestational age, maternal BMI, socioeconomic factors (social class in childhood and adult life), exercise habits, smoking status, and current lean body mass. The regression models were also investigated using a quadratic term for each of the childhood body size variables (height and weight at birth, 2, 7 and 11 years); however, there was no evidence for nonlinear associations.
We calculated z-scores (SD-score) for height, weight and BMI for each child at birth and at each birthday until 12 years of age. The z-score is the number of standard deviations by which an observation differs from the mean for the whole study group. As the children were not measured exactly on their birthdays, we obtained the z-score for birthdays by interpolating the available measurements (if a measurement had been recorded within two years of the particular age) and used these estimated values. Most of the visits to child welfare clinics occurred before age two years; fewer measurements were made between ages two years and enrolment at school. We examined the effects of gains in height, weight and BMI after birth during three periods of growth (0–2, 2–7 and 7–11 years) on CRF. In these analyses, we used the residual from linear regression where current height, weight or BMI was regressed on previous corresponding measurements. By this construction, the residuals, which we refer to as “conditional growth” are uncorrelated with the earlier size measurements and enable the effects of changes in height, weight and BMI during different growth periods to be distinguished 
The adult clinical as well as childhood growth characteristics are presented separately for men and women. Interaction terms were created to explore potential interactions between gender and growth measures in association to CRF. Since the effect of growth on CRF was similar in both, boys and girls, pooled analyses are presented.