We have retrieved weights from the Child Health records of an annual cohort of 3418 full-term children, aged 18–30 months in order to define a lower limit of normality for weight gain for both clinical and research purposes. There was a strong tendency for children at the extremes of the distribution to move inwards towards the average: only 26% of those below the 3rd centile for weight at six weeks were still below it at one year. We describe a method based on conditional standards to correct for this “regression to the mean” and produce a measure of the discrepancy between a child’s predicted and actual growth: the “thrive index”. The value of the thrive index below which only a minority of children fell provided a lower threshold for normality. While most children below the 3rd centile for weight at 9–24 months of age also had subnormal weight gain, 41 % of those with subnormal weight gain had not fallen below the 3rd centile.