From measurements to curves How to construct growth

“From measurements to curves” How to construct growth charts Tim Cole MRC Centre of Epidemiology for Child Health UCL Institute of Child Health

Methods of growth chart construction • Many different methods developed over 50 years • LMS method now widely used –British 1990 –Dutch 1997 –US CDC 2000 –WHO 2006 Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Stat Med 1992; 11: 1305 -19.

Weight in boys nonlinear age trend obvious skewness variability changes with age

LMS method • Fits growth chart centiles to skew data • Distribution at each age defined by – - Box-Cox power needed to remove skewness – - median – - coefficient of variation • L curve, M curve and S curve show , and vary with age Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Stat Med 1992; 11: 1305 -19.

L M S curves for WHO weight: 0 -5 years L M S

LMS method and z-scores • L, M and S curves estimated by penalized maximum likelihood (cubic smoothing splines) • Measurement (weight) converted to z-score using age-sex-specific values of L, M and S

When do DS infants regain bwt? • Suggestion that DS infants take longer to regain birthweight • Use weight data to investigate • Need evidence-based statement for instructions

Regaining birthweight • Not enough information to be sure • Final wording: – “Most babies lose some weight after birth, a loss up to 10% is considered acceptable. For those with Down syndrome early losses may be more than 10% and it often takes longer than 2 weeks to regain birthweight. ”

BMI in DS children • DS children more prone to overweight • BMI lookup on PCHR needs adjusting for Down Syndrome

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Weight-height conversion chart for Down Syndrome • Lower cut-offs than for PCHR • Underweight, overweight and obese 91 75 2

BMI centiles in DS girls