The one-size-fits-all fetal growth chart used by the NHS to monitor babies’ growth before birth often incorrectly classifies babies as either too small or too large, potentially missing cases that put them at risk of stillbirth or unnecessary intervention, the BMJ found today in a study of more than three million NHS births across the UK.
Fetal growth restriction (when the baby grows slower than expected in the womb) increases the risk of adverse pregnancy outcomes, so discovering this during pregnancy and before the baby is born is essential for the safety of mother and baby. Unrecognized presence during pregnancy is the most common cause of avoidable stillbirth.
NHS hospitals use different fetal weight charts to define low and high fetal weight for gestational age babies. This includes Hadlock, Intergrowth 21st (IG21), World Health Organization (WHO), and Fetal Medicine Foundation (FMF) charts, all of which cannot be adjusted. It also includes a customized GROW chart that can be customized for each mother.
Researchers from the Perinatal Research Institute in Birmingham compared the proportion of infants judged to be too small or too large based on different charts across 3.2 million births between 2015 and 2025 in 38 of England’s 42 NHS Integrated Care Boards (ICBs).
They found that using customized growth curves that adjusted for maternal characteristics such as weight and ethnic origin, which influence a baby’s normal growth in the womb, could provide more accurate and consistent data, improve safety, and allow midwives and medical staff to better identify babies who need special care during pregnancy.
Without adjusting for such factors, the proportion of babies that are too small or too large will vary widely between charts due to the different methods used for delivery. The ‘universal’ non-adjustable graphs were mainly derived from other countries, whereas the GROW standard was based on the entire UK population.
As an example, the average percentage of babies born at term (37 weeks or older) identified as small for gestational age (<10th percentile) was 4.8% for Intergrowth, 17.2% for WHO and FMF charts, and 12.3% for customized GROW charts.
For nonadjustable charts, large variations were observed even in the same chart for different ICBs due to regional population differences. We did not see this on the GROW chart as it is adjusted for normal fluctuations.
These are observations, and the researchers acknowledge that they used birth weight rather than ultrasound-estimated weight to evaluate the chart’s performance, but this allowed them to include all cases regardless of whether they had a growth scan during pregnancy.
However, this study was based on regularly collected information covering 90% of NHS areas in England, giving its conclusions greater confidence. The authors call for urgent standardization of growth curves used in the NHS.
The potential impact of using inappropriate growth curves is illustrated in a recent BMJ analysis by the Perinatal Research Institute on the need for the NHS to improve the prevention of avoidable perinatal deaths.
The authors argue that individual NHS trusts are prone to rare but devastating outcomes, which can occur as a result of local protocols and slow practice. They call for a coherent, coordinated program across the NHS that recognizes the importance of standardized developmental assessments, and suggest the establishment of real-time national oversight to monitor the quality and safety of this important area of maternity care.
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DOI: 10.1136/bmj-2026-433307
