This UK population - based study and systematic review assessed the effect of timing of folic acid supplementation during pregnancy on the risk of the baby being SGA at birth, defined as birth weight less than the 10th centile or in the lowest 10 % of babies born. (sciencedaily.com)
Scores over the 90th centile are considered clinically significant. (adc.bmj.com)
So if they're on the 25th centile for weight it means that out of 100 children yours is the 25th lightest — 75 are heavier. (bounty.com)