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Standard, Population & Customised fetal size charts 11 – Intergrowth 21

September 16, 2019

Standard charts

In 2008 the Intergrowth-21 group, funded by Bill Gates, produced a series of growth standard charts for fetuses.  Click here for their website, here for the main report and here for their estimated fetal weight standards, which were published separately. According to some historians Intergrowth-21 was originally a spin off from the World Health Organisation (WHO) fetal growth standard group. However the main reports from Intergrowth-21 preceded publication of those from WHO.

Intergrowth-21 collected scan measures from 4,600 healthy fetuses and healthy mothers in eight geographically distinct urban areas, Pelotas Brazil, Turin Italy, Muscat Oman, Oxford UK,
Seattle USA, Shunyi County Beijing China; Central Nagpur India, and Parklands Nairobi Kenya, where environmental, nutritional and social constraints on fetal growth were judged to be minimal. It was called the Fetal Growth Longitudinal Study (FGLS). They chose cities located below 1600 metres, with low levels of pollution. The women had no clinically relevant medical problems, started antenatal care before 14 weeks, had a height ≥153 cm, a body-mass index (BMI) between 18 and 30 kg/m², a haemoglobin concentration ≥110 g/L, and were not on any special diet. This resulted in a group of educated, affluent, clinically-healthy women, with adequate nutritional status, who by definition were at low risk of fetal growth restriction and preterm birth. The Intergrowth-21 group used all the latest scan methods as well as modern techniques to avoid bias (click here).

The authors found little variation by ethnicity. Specifically there were no statistically significant differences between each geographical area and the pooled data from the other seven. The charts also aligned closely with newborn charts from similar healthy populations.

This lack of important size difference between healthy fetuses from different ethnic groups implies that the differences we see every day between such groups are largely a result of environmental and nutritional factors. Once these are removed the differences disappear.  It is strong evidence against customisation by ethnicity.

But it is also the reason why some enthusiasts for customisation push back so strongly against it. They dispute the statistical methods, or point to other standard charts, notably those from WHO which we will discuss tomorrow, showing small differences between geographic groups, as evidence for customisation. But the converse argument does not apply. Finding ethnic differences even in standard charts, is not strong evidence for customisation. There are small differences, e.g between Seattle and Shunyi County, in Intergrowth-21, and between countries in WHO, but the most likely explanation is that neither group of researchers succeeded completely in removing all study participants who had environmental constraints acting on their pregnancy.

The Intergrowth-21 authors concluded that their charts are the single best growth standard chart for use worldwide, and I agree. We use them in Nottingham.

Next (click here) the WHO standard charts.

Jim Thornton


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