The Perinatal Institute gets occasional enquiries about the INTERGROWTH 21st standards for birthweight and fetal weight. We advise against their use as they can cause harm, because they
  • underestimate the diagnosis of SGA, with only about 4-5% of the maternity population identified as <10th centile - as a result of which they miss significant proportions of at-risk fetuses and infants; and
  • over-estimate the incidence of LGA, with over 20% of pregnancies considered to have a baby >90th centile, which can result in maternal anxiety and unnecessary interventions.
We are not aware of any published evidence that either the fetal weight or the birthweight standards by Intergrowth has shown any significant benefit in terms of identification of adverse pregnancy outcome. In contrast, GROW charts have been validated in a number of studies referenced in a recent review (AJOG 2018). The head-to-head comparisons with Intergrowth to date are listed here and all favour GROW. However we would be pleased to hear about any evidence to the contrary - please write to GAP Team.
E2 K2 / Athena Maternity System ‘Customised Charts’
We have been asked by NHS users of K2 Maternity Systems to look at the charts incorporated by K2 into their Athena Maternity System, which they claim to be equivalent to the Perinatal Institute’s customised charts. Our GAP and statistical teams have undertaken an analysis of these charts. They have concluded that these charts cannot be recommended for clinical use as they are untested and unvalidated, and are likely to produce the wrong cut-offs for the identification of babies at risk, with potentially severe clinical consequences. For a detailed analysis, please see here.
E3 We have implemented GROW and use customised centiles to assess whether the birthweight is SGA. However our neonatologists use the WHO standard to screen for SGA as a risk factor for hypoglycaemia. The two assessments are often out of sync. Which one should we use?
The UK-WHO standard, as used in the parent held Red Book for neonates, is an internationally derived population reference which does not adjust for individual constitutional factors that affect the normal range of birthweight, as recommended by the RCOG.

The resultant discrepancies and confusion can have negative implications for parents, professionals and the quality of care. We examined the evidence and set out the argument for standardised assessment of birthweight in paper which we submitted in May 2019 to the British Association of Perinatal Medicine (BAPM) with a request to review their recommendations.
E4 What standard for birthweight should be used for babies assessed in the Perinatal Mortality Review Tool (PMRT)?
MBRRACE have recently recommended the use of their TIMMS birthweight centile calculator based on the birthweight curves by Norris et al [1]. We examined this standard and advise against its use as 1. it is not customised, contrary to RCOG guidelines [2], and 2. it applies a neonatal (instead of fetal) weight standard to preterm births which results in substantial underestimation of SGA at preterm gestations. For further details please see our analysis here: https://bit.ly/2KV1fyM

As a general rule, it is also important that the same standard is used to assess adverse outcome as that which was applied in antenatal care. For units using GROW for assessment of fetal growth and birthweight, the same standard should be used when assessing outcome. The birthweight centile can be either calculated postnatally in the GROW App, or with the customised centile calculator https://icc.growservice.org/754731/

    1. Norris T, Seaton SE, Manktelow BN, Baker PN, Kurinczuk JJ, Field D, et al. Updated birth weight centiles for England and Wales. Arch Dis Ch - Fetal & Neonatal 2018;103(6):F577–82.
    2. Royal College of Obstetricians and Gynaecologists. The investigation and management of the small-for-gestational-age fetus. Green Top Guideline No 31 - RCOG 2013.
E5 GAP effectiveness in Scotland
Stamatina Iliodromiti, Gordon Smith et al question the usefulness of the GAP program on the basis that stillbirth rates dropped in Scotland before GAP was introduced there: https://doi.org/10.1002/uog.21999

This is a good example of how statistics can be used to fit preconceptions. The research methodology is misleading in several ways. For example, the authors curtailed the period of study to exclude the years when Scottish stillbirth rates rose again, making the apparent decline statistically insignificant. They also did not present subsequent data which showed that stillbirths fell again after the GAP program had been introduced and had time to embed in Scotland. There were other flaws in methodology and conclusions which are detailed in our response: https://doi.org/10.1002/uog.22100         
E6 The DESIGN Trial

Evaluation of GAP

Studies evaluating the effect of the GAP program or its elements have so far been observational, looking longitudinally at the effect of implementation on antenatal detection of SGA as a key indicator of quality of care 1-4. In some larger studies the effect on stillbirth rates was also evaluated 5-8 and showed a causal association and benefit. The NHS England commissioned evaluation of the Saving Babies Lives Care Bundle 9 confirmed an association between the observed decline in stillbirth rates and improved detection of SGA; most (88%) of the units providing data for this analysis were in the GAP program.

Randomised controlled trials (RCTs) are rightly considered the gold standard for evaluating different forms of management. However their applicability to evaluate a surveillance program aimed at antenatal detection of SGA, which does not prescribe actual management once SGA is detected, is debatable. Also, because of the strong element of training and protocols, randomisation has to be by units or ‘clusters’.

The DESiGN Trial was set up as a cluster RCT in London in 2016, to assess the effect of the GAP program on detection of SGA. The Perinatal Institute’s GAP team agreed to provide training and support towards implementation in units randomised to the study group. Data collection stopped in February 2019 and the study protocol 10 was also published in 2019 but at the time of this posting (March 2022), the results have not yet been published.

What we know so far: The GAP program collects routine anonymised data to provide bespoke reports to participating units. From this information it was clear that the program was not properly implemented during the time allocated for evaluation, and the study was more likely to illustrate problems with the trial than being able to produce reliable evidence on the research question. The reasons have been set out in an internal post-trial report by the GAP team available here (PDF). The DESIGN trial’s chief investigator and steering committee have been informed of these concerns in February 2021, in the hope that these issues will be considered when writing up the trial.

The Perinatal Institute encourages clinicians and researchers to undertake independent evaluations of the effectiveness of GAP within their service environment, and offers bespoke audit tools and support. For further information, please contact gap@perinatal.org.uk


  1. Gardosi J, Francis A. Controlled trial of fundal height measurement plotted on customised antenatal growth charts. BJOG. 1999;106(4):309–17. http://onlinelibrary.wiley.com/doi/10.1111/j.1471-0528.1999.tb08267.x/full
  2. Roex A, Nikpoor P, Eerd E, Hodyl N, Dekker G. Serial plotting on customised fundal height charts results in doubling of the antenatal detection of small for gestational age fetuses in nulliparous women. Australian and New Zealand Journal of Obstetrics and Gynaecology. 2012;52(1):78–82. https://doi.org/10.1111/j.1479-828X.2011.01408.x
  3. Jayawardena L, Sheehan P. Introduction of a customised growth chart protocol increased detection of small for gestational age pregnancies in a tertiary Melbourne hospital. Australian and New Zealand Journal of Obstetrics and Gynaecology. 2018;0(0). https://doi.org/10.1111/ajo.12902
  4. Cowan FJ, McKinlay CJD, Taylor RS, Wilson J, McAra‚ÄźCouper J, Garrett N, et al. Detection of small for gestational age babies and perinatal outcomes following implementation of the Growth Assessment Protocol at a New Zealand tertiary facility: An observational intervention study. Aust N Z J Obstet Gynaecol. 2020;ajo.13283: https://doi.org/10.1111/ajo.13283
  5. Gardosi J, Giddings S, Clifford S, Wood L, Francis A. Association between reduced stillbirth rates in England and regional uptake of accreditation training in customised fetal growth assessment. BMJ open 2013;3(12):e003942. http://dx.doi.org/10.1136/bmjopen-2013-003942
  6. Gardosi J, Giddings S, Buller S, Southam M, Williams M. Preventing stillbirths through improved antenatal recognition of pregnancies at risk due to fetal growth restriction. Public Health 2014 Aug;128(8). https://doi.org/10.1016/j.puhe.2014.06.022
  7. Perinatal Institute. Saving Babies in North England (SaBiNE) - Final Report 2016. https://www.perinatal.org.uk/SaBiNE_final_report_2016.pdf
  8. Hugh O, Williams M, Turner S, Gardosi J. Reduction of stillbirths in England according to uptake of the Growth Assessment Protocol, 2008-2017: 10 year population based cohort study. Ultrasound in Obstetrics & Gynecology. 2020; https://doi.org/10.1002/uog.22187
  9. Widdows K, Roberts S, Camacho E, Heazell A. Evaluation of the implementation of the Saving Babies’ Lives Care Bundle in early adopter NHS Trusts in England (SPIRE) Maternal and Fetal Health Research Centre, University of Manchester; 2018; https://bit.ly/2LZFbBv
  10. Vieira MC, Relph S, Copas A, Healey A, Coxon K, Alagna A, et al. The DESiGN trial (DEtection of Small for Gestational age Neonate), evaluating the effect of the Growth Assessment Protocol (GAP): study protocol for a randomised controlled trial. Trials. 2019 Dec;20(1):154. https://trialsjournal.biomedcentral.com/articles/10.1186/s13063-019-3242-6