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1. Main messages

This report is the first national analysis of the National Child Measurement Programme (NCMP) data which assesses how weight status changes in individual children during primary school from reception year (aged 4 to 5 years) in 2013 to 2014 to year 6 (aged 10 to 11 years) in 2019 to 2020. Only children whose measurements could be matched between those years are included (395,623 records) and clinical thresholds for child body mass index (BMI) are used for weight status.

The majority of children were a healthy weight in reception (84.7%), of these:

2. Introduction

Since the 2013 to 2014 data collection year, the NCMP IT system has collected data items (name, NHS number, date of birth and postcode) to enable the linking of a child’s measurements in reception and their subsequent measurement in year 6. In 2013 to 2014 NCMP, approximately 94% of eligible children in reception had valid measurements recorded (587,336 children). School closures in March 2020 due to the COVID-19 pandemic meant that in 2019 to 2020, the number of children measured in year 6 was lower at around 80% of previous years (491,138 children). Analysis by NHS Digital and the Office for Health Improvement and Disparities (OHID) at the time showed that the national level data was reliable and comparable to previous years. 395,623 children were matched between reception in 2013 to 2014 and year 6 in 2019 to 2020. This represented 68% of the reception cohort measured in 2013 to 2014. More detail about the sample, matching process and data quality can be found in sections 8.1 and 9 of this report.

Clinical BMI thresholds, recommended for use when assessing individual children, were used to assign children to a weight category as the analysis tracks individual children. Therefore, proportions of children within each weight category will be different to published population level prevalence estimates that use the population monitoring BMI thresholds. Further information about the clinical BMI thresholds is in the methods section of this report (section 9). For this analysis, children living with obesity are presented as 2 groups: children living with obesity excluding severe obesity and children living with severe obesity. Children living with severe obesity are at greater risk than their peers of immediate and long-term serious health conditions.

Throughout the report, statistically significant differences are determined by comparing to the average for all children (persons, England). Where a movement between weight categories is described as higher, lower, bigger, or smaller it is significantly different to the movement of all reception children who were initially in the specific weight category. However, note that not all significant differences are commented on.

The charts in this report are interactive. Users can hover over the charts to see more details and click the legend to select and de-select weight categories. More detail on how to interpret and use the charts is described in annex A. Data throughout the report is presented for children in reception moving to year 6, however data is presented for children in year 6 moving from reception in annex B as an alternative way to interpret the data. All the data used in this report and the annexes is available to download from the gov.uk website.

3. Changes in weight status

Figure 1a shows the change in child weight status from reception to year 6, the thickness of the line linking one weight category to another represents the number of children moving between the weight categories. Only lines where 3% or more of children moved out of a weight category in reception are shown on the chart. Figure 1b presents the same data as Figure 1a as a bar chart for direct comparison (all groups are shown).

In England:

Looking in more detail at children who were not a healthy weight in reception shows that:

These next two points look at movement between the obesity categories:

Figure 1a: Changes in child weight status between reception aged 4 to 5 years and year 6 aged 10 to 11 years

Movements below 3% are excluded from figure 1a and this has meant that not all movements into underweight from a different category are shown. Figure 1b, below, shows all movements between BMI groups regardless of size.

Figure 1b: Changes in child weight status between reception aged 4 to 5 years and year 6 aged 10 to 11 years

The rest of this report looks at how these patterns in weight change vary between different demographic groups. Bar charts are used in the remainder of the report as they make it easier to compare movements between groups, for example between boys and girls, than comparing two Sankey diagrams (flow diagram as shown in figure 1a). However, they do not show the number of children in different weight categories so comparisons back to figure 1a are useful as a reminder that the majority of children are in the healthy weight category followed by the overweight and obesity excluding severe obesity categories. The severe obesity and underweight categories contain the smallest number of children.

4. Changes in weight status by sex

The patterns of change in weight categories between reception and year 6 are different between boys and girls. Figure 2 shows the changes in child weight status by sex compared with all children in this analysis:

Figure 2: Changes in child weight status between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by sex

5. Changes in weight status by ethnic group

There are known disparities in weight status by ethnicity, with children from black and some Asian ethnic groups having higher prevalence of obesity than children of white British ethnicity. Data is presented here for the 6 ethnic groups of Bangladeshi, black African, black Caribbean, Indian, Pakistani, and white British children.

Figure 3 shows how patterns of change between weight categories differ by ethnic group.

Compared with all children who were underweight in reception:

Compared with all children who were a healthy weight in reception:

Compared with all children who were overweight in reception:

Compared with all children who were living with obesity (excluding severe obesity) in reception:

Compared with all children who were living with severe obesity in reception:

Figure 3: Changes in child weight status between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by ethnic group

6. Changes in weight status by deprivation

There is a strong correlation between weight status and deprivation, with children living in the more deprived areas experiencing a higher prevalence of obesity. Data is presented using the Index of Multiple Deprivation (IMD), a measure of relative deprivation for small areas. The small areas are divided according to their deprivation rank into 5 equal groups (quintiles), ranging from the most deprived areas (quintile 1) to the least deprived areas (quintile 5). Only quintiles 1 and 5 have been commented on in this report, but data for all quintiles is available in Figure 4 and the data download.

Compared with all children who were a healthy weight in reception:

Compared with all children who were living with obesity (excluding severe obesity) in reception:

Compared with all children who were living with severe obesity in reception:

Figure 4: Changes in child weight status between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by deprivation quintile

7. Changes in weight status by region of residence

This analysis uses English region of residence when a child was in reception. Only regions where a proportion of children moving between categories was higher than the England average are commented on. However, movements for all regions can be seen in the Figure 5 and the data download.

Compared with all children who were a healthy weight in reception:

Compared with all children who were living with obesity (excluding severe obesity) in reception:

Compared with all children who were living with severe obesity in reception:

Data at sub national level needs to be rounded and suppressed. Therefore, not all movements between categories are shown in figure 5 and proportions may not also sum to 100 due to rounding. Where this has happened, the bars which are on the chart have been rescaled to add to 100 but the real values can be seen by hovering over the bars or in the data download.

Figure 5: Changes in child weight status between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by region of child residence

8. Conclusion

This report is currently the largest tracking analysis that uses NCMP data. The findings therefore provide a new insight into how weight status tracks during primary school and is an important benchmark for future analyses.

In this analysis, most children who start primary school with a healthy weight retain this healthy weight status until the end of primary school. However, the flow of children from a healthy to an unhealthy weight status (underweight, overweight or living with obesity) was larger than the flow of children from an unhealthy to a healthy weight. This is reflected in the increase in obesity prevalence seen between reception and year 6.

This report has shown that clear demographic and geographic disparities are evident in the change of weight status of children during the primary school years. The proportion of children moving from a healthy weight category to a higher weight category was generally higher for those who were either boys, from a black or minority ethnic group, living in more deprived areas, or living in either the North, Midlands or London.

8.1 Limitations of this analysis

All the data analysed in this report were collected before the COVID-19 pandemic and therefore it is not known if some of the findings have been affected by the pandemic.

The 2019 to 2020 NCMP data collection year was directly impacted by the COVID-19 pandemic. Not all children in year 6 were measured as schools were closed from 20 March 2020. Therefore, the number of children used in this analysis (395,623) is smaller than would be used in a year not affected by the pandemic, so the results presented in this report are not as robust as they would be normally.

Despite the smaller than usual data collection in 2019 to 2020 it was still possible to link 68% of reception records collected in 2013 to 2014 with the measurement data in year 6. Comparison of the samples of linked and unlinked records has shown that the samples are similar, however there are some small demographic differences, see data tables in annex C. This should be borne in mind when reading the findings of this report as those with linked measurements are a slightly biased subset of all reception measurements.

Some of the differences between ethnic groups and regions could be due to confounding factors such as area level deprivation.

8.2 Future tracking analysis

As more data becomes available it will be possible to look at how these patterns of child weight change over time and will enable any changes in disparities to be monitored.

As more data is collected it will be possible to look at data by local authority, where numbers measured allow, that will enable more granular analysis.

9. Methods and further information

9.1 Record matching and data quality

Record matching was completed by NHS Digital using a combination of NHS number, forename, surname, gender, postcode, and date of birth. The matching method applied by NHS Digital changed between academic year 2013 to 2014 and academic year 2019 to 2020 but used similar data items for matching. For the academic year 2013 to 2014 matching method, the minimum amount of data required for a match to be confirmed was either:

  • NHS number and date of birth, or
  • surname, forename, and date of birth

For the academic year 2019 to 2020 matching method, the minimum amount of data required for a match to be confirmed was either:

  • NHS number and date of birth, or
  • surname, gender and date of birth

NHS Digital provided a linking file which allowed OHID to link 2013 to 2014 NCMP data with 2019 to 2020 NCMP data, without receiving any of the above listed data items. The chance of a match between records is increased if more data items are provided, particularly if this includes NHS number.

NHS numbers are theoretically unique to an individual unlike names and postcodes, and by including them as part of the matching process it improves the quality of a match although it is possible to produce a match without them. NHS number collection as part of the NCMP has increased overtime; in 2013 to 2014, 38.5% of records collected were missing NHS number, in 2019 to 2020 this had reduced to 18.2% of records. All matched records are used in this analysis regardless of whether they were matched using NHS number or not.

Initially 399,023 records were in the NCMP for children that attended a mainstream state-funded school in England with measurements recorded in both 2013 to 2014 and 2019 to 2020 data sets. Some children had multiple matches to records in the dataset, and this will mainly occur when a child has moved school and been measured twice although some may be data entry errors. In instances where children had multiple matches the most recent record with a valid measurement was used in the analytical dataset. Removing multiple matches reduced the number of matched records to 397,333.

As child weight category is defined using sex specific growth charts, records where the sex recorded changed between reception and year 6 were removed from the dataset, reducing the count to the final data set of 395,623 records.

Eighty-one percent of eligible records for children measured in year 6 were matched to their measurement record in reception in 2013 to 2014.

For analysis by ethnicity, the ethnicity recorded in year 6 was used as it was the most recently recorded. If the ethnicity in year 6 was not stated or missing, then the ethnicity from reception was used instead:

  • 5.0% of matched records (19,792) had no ethnicity recorded in either reception or year 6 (includes not stated and missing records)
  • For the ethnic groups in this report, 3.8% of matched records had a different ethnic group in year 6 compared to reception (excluding records where ethnic group was not stated or missing in either reception or year 6)
  • 17.4% of reception records had no ethnicity recorded, which decreased to 12.2% by year 6, suggesting an improvement in data collection over time

For analysis by deprivation and region, the Lower Super Output Area (LSOA) recorded in reception was used to assign each of the geographical data items. This ensured any interventions delivered in response to the child’s weight status in reception were more likely to be allocated to the area where the child was living when the intervention took place. If the LSOA in reception was invalid or unknown, the LSOA from year 6 was used to assign the geographical data item instead:

  • 4.5% of matched records had a different region recorded in reception compared to year 6
  • deprivation quintile was different between reception and year 6 for 24.9% of records
  • geographical data items were missing in 0.4% of reception records and 0.1% of year 6 records

A very small number (less than 0.01%) of matched records appeared to show movement from underweight to living with severe obesity and vice versa. Whilst this is not impossible there is a possibility that these will be due to a data quality issue. These movements are not commented on in this report as the numbers of children who move between these weight categories is very small, but they are available in the data download.

9.2 Determining significant differences

Significance tests have been used in this report to determine whether differences between prevalence estimates are statistically significant.

When the confidence intervals do not overlap the differences are considered as statistically significantly different. However, in some cases, estimates with overlapping confidence intervals will still be statistically significantly different. Where confidence intervals overlap with the confidence intervals of the average value, additional significance testing has been applied using the approach applied by NHS Digital in the annual NCMP report, the method is outlined in Annex F of the NHS Digital report appendices.

9.3 Confidence intervals

A confidence interval is a range of values that is used to quantify the imprecision in the estimate of a particular indicator. Specifically, it quantifies the imprecision that results from random variation in the measurement of the indicator. A wider confidence interval shows that the indicator value presented is likely to be a less precise estimate of the true underlying value. Statistical significance was determined by non-overlapping confidence intervals.

Confidence intervals were calculated using the Wilson Score method which gives very accurate approximate confidence intervals for proportions and odds based on the assumption of a Binomial distribution. It can be used with any data values, even when the denominator is very small and, unlike some methods, it does not fail to give an interval when the numerator count, and therefore the proportion, is zero. The Wilson Score method is the preferred method for calculating confidence intervals for proportions and odds. The method is described in detail in APHO Technical Briefing 3: Commonly used public health statistics and their confidence intervals

9.4 Body mass index (BMI) clinical classification definitions

Assessing the BMI of children is more complicated than for adults because a child’s BMI changes as they mature. Growth patterns differ between boys and girls, and both the age and sex of a child needs to be considered when estimating BMI.

In England the British 1990 growth reference (UK90) for BMI is recommended for use to determine weight status according to a child’s age and sex. Each child’s BMI is calculated and compared with the BMI distribution for children of their age and sex from the UK90 growth reference. Clinical thresholds were used in this report as the data tracks individual children. For clinical assessment BMI is classified according to the following list using the UK90 growth reference:

  • underweight: less than the 2nd centile
  • healthy weight: greater than or equal to the 2nd centile and less than the 91st centile
  • overweight: greater than or equal to 91st centile
  • obesity: greater than or equal to 98th centile
  • severe obesity: greater than or equal to 99.6th centile

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