C H A P T E R 6 lands. All child records of children, born in 2002, who developed overweight at the age of 5 years, were collected. Those children were examined by a youth health care physician at the age of 5 years, in 2007. During this examination, the presence of overweight or obesity was determined, according to Dutch cut-‐off points for the Body Mass Index (BMI) in children defined by Hirasing et al6. We collected all growth data of these children, from birth until the age of 5 years. Data of length, weight, gender and gestational age at birth were collected. The total amount of children included is 120 for the group of children with overweight or obesity at the age of 5 years (overweight group). As a control group, we also col-‐ lected data of 120 children, born in 2002, in the same region, who did not develop overweight or obesity at the age of 5 years. Those children were selected at ran-‐ dom. Because the exact values of birth weight and birth length are needed to use the prediction model for the Ponderal Index (PI), children whose birth length was not known were excluded. Regarding this, the total amount of children in the over-‐ weight group is 109 and in the control group 111. Because not every child is meas-‐ ured at the exact moment of 1 year, the range of time that is used is: 0.9-‐1.2 year for the measurements of 1 year. We already constructed a prediction model for abnormal growth based on longitu-‐ dinal data of a reference population, consisted of 372 infants, measured from 1995-‐ 1999 in a longitudinal growth study, in the South of the Netherlands7. In this model the prediction is based on the calculation of the PI. The PI is defined as: PI = 100 × weightgr 92 ! heightcm The PI yields valid results even for very short and very tall person9. Because of this property, it is most commonly used in pediatric10. In our prediction model, the individual growth of a child was analyzed. For every child PI values were plotted against age, until the age of 1 year. By using regression analysis the individual α (αPI ind) was calculated, representing the individual change of growth of a child. Next, all αPI ind values were plotted against all individual values of PI at birth. The regression line was calculated again. Based on the new corre-‐ sponding α values, representing the total child population (αPI tot), the expected value of the PI (PIe) at a certain age (t) can be estimated, depending on the child’s starting position, according to the formula: PIe = (ax + b) * t + x
Proefschrift binnenwerk Manon Ernst_DEF.indd
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