Changes in BMI category of children between the first and final years of primary school, 2023 to 2024
1. Main messages
This report presents analysis examining how primary school children have moved between body mass index (BMI) categories from reception (age 4 to 5 years) to year 6 (age 10 to 11 years). This report links records for children who were in reception in 2017 to 2018 and in year 6 in 2023 to 2024. Clinical thresholds are used to categorise child BMI1.
In summary, more children moved from a healthy weight in reception to a higher BMI category (overweight, living with obesity or severe obesity) in year 6 (91,803 which is 17.6% of all children) than the number who moved the other way from a higher BMI category in reception to a healthy weight in year 6 (17,144, 3.3% of all children). This is reflected in an increase in the proportion of children living with obesity between reception and year 6.
The other findings in this summary concentrate on the proportion of children who were a healthy weight in reception and whether they remained a healthy weight in year 6 or moved to a higher BMI category.
The majority of children in reception were a healthy weight (84.8%) in 2017 to 2018, of these:
- the majority (77.6%) remained a healthy weight in year 6, when they were measured again in 2023 to 2024; this proportion was higher among girls (79.7%) than boys (75.5%) and among children living in the least deprived areas (84.7%) than the most deprived (71.8%)
- the proportion remaining a healthy weight in year 6 was also higher among White British children (79.1%), White and Asian children (79.8%) and Chinese children (80.1%), and among children from the East of England (79.3%), South West (79.8%) and South East (79.9%)
- 20.8% moved from a healthy weight to a higher BMI category; this proportion was higher among boys (23.1%) than girls (18.4%), higher among children living in the most deprived areas (26.4%) than the least deprived (13.4%), and higher among children from Asian and Black ethnic groups
This report updates the previous analysis which was based on the pre-pandemic cohort who were in reception in 2013 to 2014 and year 6 in 2019 to 2020. Compared with this earlier cohort:
- a lower proportion of children remained a healthy weight between reception and year 6 (77.6% compared with 78.8%)
- a higher proportion of children moved to a higher BMI category between reception and year 6 (20.8% compared with 19.7%)
2. Introduction
This report is the second national analysis of the National Child Measurement Programme (NCMP) data which assesses how BMI category has changed in individual children during primary school from reception year (aged 4 to 5 years) to year 6 (aged 10 to 11 years). The first report was published in 2022.
National, regional and local authority data is analysed in this report and presented in the obesity profile. Data presented in the obesity profile combines 3 annual cohorts of NCMP data to deliver robust estimates at local authority level, and therefore, England prevalence and inequalities analysis in the obesity profile will have different values to those presented in this report which are based on the latest cohort.
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.
Clinical BMI thresholds, recommended for use when assessing individual children, were used to assign children to a BMI category as the analysis tracks individual children. Therefore, proportions of children within each BMI category will be different from 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 (chapter 7). 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 in England. Where a movement between BMI categories for a subgroup of children is described as higher, lower, bigger, or smaller it is significantly different from the same movement for all reception children. However, note that not all significant differences are commented on but are included in the spreadsheet published with this report.
The charts in this report are interactive. Users can hover over the charts to see more details. Data throughout the report is presented for children in reception who have moved to year 6. All the data used in this report is available to download from the gov.uk website.
3. Changes in BMI category
Figure 1a and 1b show the change in child BMI category from reception to year 6. Figure 1a shows the data as a bar chart, and figure 1b shows the flow of movement between BMI categories between reception and year 6. The thickness of the line linking 1 BMI category to another represents the number of children moving between the BMI categories. Only lines where 200 or more children moved out of a BMI category in reception are shown on figure 1b. Two hundred measurements equates to around 0.05% of all measurements. This simplifies the chart and makes it easier to see movements which are experienced by larger groups of children.
In England:
the majority of children were a healthy weight in reception (84.8%); 77.6% of these children remained a healthy weight in year 6, and 20.8% moved to a higher BMI category
of the 14.1% of children who were in a higher BMI category the majority (76.6%) remained in a higher BMI category and 23.4% moved to a healthy weight
Figure 1a and 1b: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years
Movements with less than 200 measurements are not shown on this chart. 200 measurements equates to less than 0.05% of all measurements.
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 2 Sankey diagrams (flow diagram as shown in figure 1b). However, they do not show the number of children in different BMI categories so comparisons back to figure 1b 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 BMI category by demographics
This section of the report looks at how patterns of change in BMI category varied between different demographic groups for children who were in the latest cohort of children and were in reception (aged 4 to 5 years old) in 2017 to 2018 and in year 6 (aged 10 to 11 years old) in 2023 to 2024.
4.1 Sex
The patterns of change in BMI categories between reception and year 6 were different between boys and girls. Figure 2 shows the changes in child BMI category by sex compared with all children in this analysis:
- a larger proportion of girls (79.7%) than boys (75.5%) who were a healthy weight in reception remained a healthy weight in year 6, and a lower proportion of girls (18.4%) than boys (23.1%) moved to a higher BMI category
Figure 2: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by sex
4.2 Ethnic group
There are known inequalities in BMI category 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 17 ethnic groups recorded in the NCMP data. Figure 3a to figure 3e shows how patterns of change between BMI categories differ by ethnic group. Users can click on the tabs above the chart to view the data for each BMI category.
Compared with all children who were a healthy weight in reception, higher proportions of children remained a healthy weight for the following ethnic groups:
- Chinese (80.1%)
- White and Asian (79.8%)
- White British (79.1%)
Compared with all children who were a healthy weight in reception, higher proportions of children moved from healthy weight to a higher BMI category for the following ethnic groups:
- Bangladeshi (27.8%)
- Indian (22.4%)
- Pakistani (26%)
- Any other Asian background (23.9%)
- Black African (26.5%)
- Black Caribbean (27.2%)
- Any other Black background (26.7%)
- White and Black African (24.9%)
- White and Black Caribbean (26.2%)
- Any other ethnic group (24%)
Figure 3a to 3e: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by ethnic group
Figure 3a: Changes in weight status for healthy weight children in reception by ethnic group
Figure 3b: Changes in weight status for children living with severe obesity in reception by ethnic group
Figure 3c: Changes in weight status for children living with obesity (excluding severe obesity) in reception by ethnic group
Figure 3d: Changes in weight status for overweight children in reception by ethnic group
Figure 3e: Changes in weight status for underweight children in reception by ethnic group
4.3 Deprivation
There is a strong correlation between BMI category 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 10 equal groups (deciles), ranging from the most deprived areas (decile 1) to the least deprived areas (decile 10). Only the most and least deprived areas have been commented on in this report, but data for all deciles is available in figure 4a to 4e and the spreadsheet published with this report.
Compared with all children who were a healthy weight in reception:
- Higher proportions of children living in the least deprived areas (84.7%) and lower proportions of children living in the most deprived areas (71.8%) remained healthy weight in year 6
- higher proportions of children living in the most deprived areas (26.4%) and lower proportions of children living in the least deprived areas (13.4%) moved to a higher BMI category
Figure 4a to 4e: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by deprivation decile
Figure 4a: Changes in weight status for healthy weight children in reception by deprivation
Figure 4b: Changes in weight status for children living with severe obesity in reception by deprivation
Figure 4c: Changes in weight status for children living with obesity (excluding severe obesity) in reception by deprivation
Figure 4d: Changes in weight status for overweight children in reception by deprivation
Figure 4e: Changes in weight status for underweight children in reception by deprivation
4.4 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 figure 5 and the spreadsheet published with this report.
Compared with all children who were a healthy weight in reception:
- higher proportions of children from the East of England (79.3%), South West (79.8%) and South East (79.9%) remained a healthy weight in year 6
- higher proportions of children from the North East (22.1%), North West (21.4%), Yorkshire and the Humber (22.6%), West Midlands (23.1%), and London (21.6%) moved to a higher BMI category in year 6
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 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 and in the spreadsheet published with this report.
Figure 5a to 5e: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by region of child residence
Figure 5a: Changes in weight status for healthy weight children in reception by region
Figure 5b: Changes in weight status for children living with severe obesity in reception by region
Figure 5c: Changes in weight status for children living with obesity (excluding severe obesity) in reception by region
Figure 5d: Changes in weight status for overweight children in reception by region
Figure 5e: Changes in weight status for underweight children in reception by region
5. Changes in BMI category by cohort year
With 4 years of data linking children’s measurements in reception and their subsequent measurement in year 6, patterns of change between annual cohorts can be analysed.
Reception and year 6 data was linked for the following measurement years:
- reception in 2013 to 2014 and year 6 in 2019 to 2020
- reception in 2015 to 2016 and year 6 in 2021 to 2022
- reception in 2016 to 2017 and year 6 in 2022 to 2023
- reception in 2017 to 2018 and year 6 in 2023 to 2024
Only a small number of children were measured in year 6 in 2020 to 2021 so the reception year cohort in 2014 to 2015 is not included in this report (see chapter 7 for more details).
Figure 7 shows the patterns of change over time:
- the percentage of children remaining a healthy weight was highest in the pre-pandemic 2019 to 2020 year 6 cohort (78.8%), this decreased to 76.0% in the 2021 to 2022 year 6 cohort. It then increased in the later cohorts to 77.1% in 2022 to 2023, and to 77.6% in 2023 to 2024 but remained below the pre-pandemic level
- the percentage of children moving from a healthy weight to a higher BMI category was lowest in the 2019 to 2020 year 6 cohort (19.7%) and highest in the 2021 to 2022 year 6 cohort (22.6%), the percentage then decreased to 21.3% in the 2022 to 2023 year 6 cohort and 20.8% in 2023 to 2024 but remained higher than the pre-pandemic level
Figure 7: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years by cohort year
The rest of this report looks at how these patterns in weight change varied between different demographic groups for children who were in the latest cohort of children and were in reception (aged 4 to 5 years old) in 2017 to 2018 and in year 6 (aged 10 to 11 years old) in 2023 to 2024.
6. Conclusion
This report is the largest analysis of linked NCMP data and the first to provide data for all local authorities in England. The findings therefore provide further insight into how children move between BMI categories during primary school across the country and is an important benchmark for future analyses. In this analysis, most children who start primary school with a healthy weight retain this healthy BMI category until the end of primary school. However, the flow of children from a healthy to a higher BMI category was larger than the flow of children from a higher BMI category to a healthy weight. This is reflected in the increase in obesity prevalence seen between reception and year 6.
Overall patterns of movement between different BMI categories are similar in the cohort groups examined. There was some change in the patterns of movement between BMI categories over time, but the changes were small. The annual reporting of child weight data at a population level, shows that obesity prevalence in year 6 has decreased since it peaked in 2020 to 2021, however it remains higher than pre-pandemic levels and is in line with the increasing pre-pandemic trend. The proportion of children in reception living with obesity remains in line with the stable pre-pandemic level.
This report has shown that clear demographic and geographic inequalities are evident in the change of BMI category of children during the primary school years. The proportion of children moving from a healthy to a higher BMI category was generally higher among boys or those who were either from a Black or Asian ethnic group, living in more deprived areas, or living in either the North, Midlands or London.
6.1 Limitations of this analysis
There may be a bias in the matched records. For England 85.8% of reception measurement records matched to a year 6 measurement record, but across the country percentage of matched records varies by local authority.
7. Methods and information
7.1 Record matching process
Record matching was completed by NHS England using a combination of NHS number, forename, surname, sex, postcode, and date of birth. The matching method applied by NHS England changed between reception measurements prior to academic year ending 2020 but used similar data items for matching. For the academic years 2013 to 2014 to 2017 to 2018 matching method (reception measurements), 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 to 2023 to 2024 matching method (year 6 measurements), the minimum amount of data required for a match to be confirmed was either:
- NHS number and date of birth, or
- surname, sex and date of birth
NHS England provided a linking file which allowed OHID to link NCMP data between measurement years, 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 and by 2023 to 2024 this had reduced to 9.7%. There remains variation at a local authority level and a number of local authorities have 100% missing NHS number. Data on missing NHS number by local authority is available from the data quality information collected by NHS England. All matched records are used in this analysis regardless of whether they were matched using NHS number or not.
7.2 Data cleaning for the report analysis
There were 525,474 records in the NCMP dataset for children that attended a mainstream state-funded school in England with measurements recorded in both 2017 to 2018 and 2023 to 2024 data sets. Some children had multiple matches to records in the dataset, this will mainly occur when a child has moved school and been measured twice although some may be data entry errors. The most recent record with a valid measurement was used in the analytical dataset where multiple matches occurred. Removing multiple matches reduced the number of matched records by 1,436 to 524,038.
Sex of child
As child BMI category is defined using sex specific growth charts, records where the sex recorded was different between reception and year 6 were removed from the analysis dataset. This removed 2,704 records reducing the count to the final data set of 521,334 records.
Eighty five percent of eligible records for children measured in reception were matched to their measurement record in year 6 in 2023 to 2024. The reasons why a child may not have a measurement record in year 6 include, but is not limited to:
- child was absent on day of measurement
- child left mainstream education
- child left the country
- child did not want to be measured
- parents of child refused consent to measure
- school did not participate in the NCMP
Ethnicity of child
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:
- 3.8% of matched records had no ethnicity recorded in either reception or year 6 (includes not stated and missing records)
- for the ethnic groups in this report, 7.4% 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)
- 14.1% of reception records had no ethnicity recorded, which decreased to 9.5% by year 6, suggesting an improvement in data collection over time
Lower Super Output Area (LSOA) of child
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 BMI category in reception were more likely to be allocated to the area where the child was living when the intervention took place.
For deprivation analysis, if the LSOA in reception was invalid or unknown, the LSOA from year 6 was used to assign the IMD decile:
- deprivation decile was different between reception and year 6 for 28.7.% of records
For regional and local authority analysis, if the LSOA in reception was invalid or unknown, the LSOA from year 6 was used to assign the region or local authority. If both the LSOA in reception and year 6 were invalid or unknown then the LSOA of the school at reception was used to assign region or local authority:
- 4.2% of matched records had a different region recorded in reception compared to year 6
LSOA was missing in 0.2% of reception records and 0.1% of year 6 records; this meant that local authority, region or deprivation decile was also missing for these records. Region and deprivation differences may be due to the child changing address between reception and year 6.
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, it may 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 BMI categories is very small, but they are available in the data download.
Cohort year
Whilst the latest years of data can be considered complete collection years, 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 the analysis (395,623) for this cohort is smaller than the other years in the analysis not affected by the pandemic. The results presented in this report for the 2019 to 2020 NCMP data are not as robust as the more complete collection years, but analysis by NHS England 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.
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. For reception records collected in 2015 to 2016, 83.9% were linked with year 6 records, for reception records in 2016 to 2017 85.3% were linked and for reception records collected in 2017 to 2018 85.8% of records were linked to a valid year 6 record.
2020 to 2021 data is not included in the cohort analysis as only around 20% of the usual sample was collected.
7.3 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.
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 (PDF 571KB)
7.4 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 England in the annual NCMP report, the method is outlined in Annex F of the NHS England report appendices (PDF 393KB).
7.5 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 interpreting BMI.
In England the British 1990 growth reference (UK90) for BMI is recommended for use to determine BMI category 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
Population prevalence of overweight and obesity using NCMP data is reported using the population monitoring thresholds of the UK90 growth reference to classify a child’s BMI. For population monitoring purposes, a child’s BMI is classed as overweight or obese where it is on or above the 85th centile or 95th centile, respectively, based on the UK90 growth reference data.
The population monitoring cut offs for overweight and obesity are lower than the clinical cut offs (91st and 98th centiles for overweight and obesity) used to assess individual children; this is to capture children in the population in the clinical overweight or obesity BMI categories and those who are at high risk of moving into the clinical overweight or clinical obesity categories. This helps ensure that adequate services are planned and delivered for treatment and prevention of obesity.
If you have any queries about this report, please email: PHA-OHID@dhsc.gov.uk
Footnotes
Clinical thresholds are different to most published reports which use population health monitoring cut offs↩︎