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Assessing the optimal time interval between growth measurements using a combined data set of weights and heights from 5948 infants
  1. Charlotte Margaret Wright1,
  2. Caroline Haig2,
  3. Ulla Harjunmaa3,
  4. Harshine Sivakanthan4,
  5. Tim J Cole5
  1. 1Department of Child Health, School of Medicine, Nursing and Dentistry, University of Glasgow, Glasgow, UK
  2. 2Robertson Centre for Biostatistics, Institute of Health and Wellbeing, University of Glasgow, Glasgow, UK
  3. 3Center for Child, Adolescent and Maternal Health Research, Faculty of Medicine, Tampere University, Tampere, Finland
  4. 4Department of Human Nutrition, School of Medicine, Nursing and Dentistry, University of Glasgow, Glasgow, UK
  5. 5Population Policy and Practice department, UCL GOS Institute of Child Health, London, UK
  1. Correspondence to Professor Charlotte Margaret Wright, Royal Hospital for Children, Office Block CO/2, QE Hospital Campus, Govan, Glasgow G51 4TF, UK; charlotte.wright{at}


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
  • growth
  • primary health care
  • statistics

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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.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

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