Background Current guidance on the optimum interval between measurements in infancy is not evidence based. We used routine data to explore how measurement error and short-term variation (‘noise’) might affect interpretation of infant weight and length gain (‘signal’) over different time intervals.
Method Using a database of weights and lengths from 5948 infants aged 0–12 months, all pairs of measurements per child 2, 4 and 8 weeks apart were extracted. Separately, 20 babies aged 2–10 months were weighed on six occasions over 3 days to estimate the SD of the weight difference between adjacent measurements (=116 g). Values of 116 g and 0.5 cm for ‘noise’ were then used to model its impact on (a) the estimated velocity centile and (b) the chance of seeing no growth during the interval, in individuals.
Results The average gain in weight and length was much larger than the corresponding SD over 8-week and 4-week time intervals, but not over 2 weeks. Noise tended to make apparent velocity less extreme; after age 6 months, a 2-week velocity that appeared to be on to the ninth centile, would truly be on the second–third centile if measured with no noise. For 2-week intervals, there was a 16% risk of no apparent growth by age 10 months.
Conclusions Growth in infancy is so rapid that the change in measurements 4–8 weeks apart is unlikely ever to be obscured by noise, but after age 6 months, measurements 2 weeks or less apart should be treated with caution when assessing growth faltering.
- child health
- primary health care
Data availability statement
Data are available upon reasonable request. The source data used for this analysis could be made available on reasonable request to the authors.
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Contributors CMW conceived the study, led the analysis and drafted the paper. CH extracted the data for the main analysis and undertook the basic analysis. UH collected the Finnish data, advised on its use and commented on the draft. HS ran the weighing study and undertook the literature review. TJC advised on the design, undertook the main analysis and contributed to the paper drafting.
Funding This work was supported by Chief Scientist Office, Scotland (90549) and Glasgow Children’s Hospital Charity (168979-01).
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.
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