On Foetal Growth Charts – the WHO May Have Adopted the Correct Policy for the Wrong Reason

The journal ‘Science’ reports a controversy over two studies of foetal growth across countries [1] – the first study showed very similar growth rates across eight countries (Brazil, Italy, Oman, UK, USA, China, India, Kenya).[2] They conclude that a common threshold should be used in countries to identify slow-growing foetuses. The second study looks only at the socio-economically advantaged populations across ten countries, ranging from Norway and Denmark to India and Egypt.[3] It finds markedly different rates across countries among socio-economically advantaged segments of the population. So that would suggest the use of country-specific thresholds.
I am not so sure – I question the assumption that the search for the growth-retarded foetus should be based on a fixed proportion of the foetal population – say the slowest growing 5%. The risk of stillbirth is higher in the countries with slower foetal growth (e.g. India and Egypt), than in those with higher growth rates (e.g. Norway and Denmark). So the cut-off threshold for foetal growth as a screening test should, logically, be set at a higher point in high-risk countries than in lower-risk countries. If it is set to identify the ‘bottom’ 5% in low-risk countries it should be set at, say, 10% in high-risk countries. This suggests that the WHO (which recommends a universal chart on the basis of the first study above) has the correct solution for the wrong reason. The universal chart will identify a higher proportion of still-births in the high-risk countries – just what one would want.

— Richard Lilford, CLAHRC WM Director

References:

  1. de Vrieze J. Big studies clash over fetal growth rates. Science. 2017; 355(6323): 336.
  2. Papageorghiou AT, Ohuma EO, Altman DG, et al. International standards for fetal growth based on serial ultrasound measurements: the Fetal Growth Longitudinal Study of the INTERGROWTH-21stProject. Lancet. 2014; 384: 869-79.
  3. Kiserud T, Piaggio G, Carroli G, et al. The World Health Organization Fetal Growth Charts: A Multinational Longitudinal Study of Ultrasound Biometric Measurements and Estimated Fetal Weight. PLoS Medicine. 2017.
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