Current charts are UK-WHO, which is to say a combination of UK growth cohorts with world wide cohorts. This is to correct for the low prevalence of breast feeding and high prevalence of obesity in the UK, and assumes that there is little genetic differences in growth.
Between 37 and 42 get plotted as term. No centiles for first 2 weeks as dip expected. One in 5 still below birth weight at 2 weeks, only one in 50 will be 10% below or more. Still mainly well, but suggests feeding problem worthy of further assessment, and in a few there will be an otherwise occult pathological condition eg cardiac or metabolic disorder.
Preterms get plotted on both Preterm section of chart AND day 0! Because assessing early growth works best on day 0 centile, whereas later growth more likely to be related to preterm centile. Plot after birth on preterm section until you hit the end (42 weeks) then continue on to main 0-1yr chart, plotting a point for calendar age but adding arrow indicating gestational age. Otherwise unclear whether corrected or not.
Children up to age 2 get weighed without clothes or nappy. From 2, minimal clothing and no shoes. Height is hard! Only act on several measurements that appear consistent!