INTERGROWTH-21st - Time to standardise fetal measurement in Australia.

Abstract

Fetal measurements have been a contentious issue in ultrasound departments worldwide for decades. With a plethora of literature on measuring planes, mathematical formulae, local charts, and differences in fetal size due to various factors, deciding which charts to use can be confusing. Australia has not been immune to this issue. In 2001, ASUM recommended the use of the Campbell Westerway charts for CRL, BPD, OFD, HC, AC, FL & HL, which were formulated from an Australian population. Despite this recommendation, there are currently at least eight fetal growth charts in clinical use around Australia. 1 Unfortunately, it is not uncommon to find different charts on different machines within the same department, which is confusing for follow‐up growth studies, and affects parents, clinicians and researchers alike. For example, a baby diagnosed as small for gestational age (SGA) by one practitioner is later that day found to be ‘normal’ with a different reference chart at the referral hospital. This makes counselling challenging: why does this difference exist? Is it due to the chart or the quality of scanning? Is the baby at risk or not? The issue illustrates the problem of a lack of uniform ultrasound reporting, which may be particularly noticeable in countries like Australia where scans are commonly performed in small private practices. In general terms, fetal growth can be assessed using charts that are: 1 Derived from the observed distributions of fetal size for gestational age in a defined population 2 Customised on the basis of maternal characteristics such as maternal parity, height and ethnicity including an estimate of fetal weight based on Hadlock's proportionality equation 3 Standards derived from a healthy population purposely selected to reflect optimal growth, based on observed measurements of fetuses that are free from adverse constraints on growth and which are independent of time or place. SGA is most commonly defined as the 10th centile of estimated fetal weight (EFW) or abdominal circumference (AC). It must be realised that the apparent ‘prevalence’ of SGA will always be close to the 10th centile when reference or customised charts are used. This is despite the fact that the prevalence of other perinatal conditions differs greatly around the world: for example, differences in rates of pre‐eclampsia or gestational diabetes are readily accepted, without a call for local definitions, demonstrating the illogical nature of insisting on an SGA prevalence that is “fixed” at 10%. So which fetal biometry charts should be used from the hundreds available around the world? After over five decades of obstetric ultrasound there has been no implementation of an international standard. Contrast this with the consensus on optimal growth in paediatrics. Since the 1970s, it has been observed that growth in children depends more on their environment and nutritional state, than ethnic origin. 2 In 1996, the WHO Multicentre Growth Reference Study (MGRS) was established to prove whether this hypothesis was indeed correct for babies born in diverse populations around the world. Across six countries, researchers followed the growth and development of 8406 healthy, breast‐fed babies until 5 years of age. 3 They demonstrated that, under such conditions, growth was remarkably similar in childhood. 4 This led to the release of the WHO Child Growth Standards in 2006 and these have subsequently been adopted in over 130 countries. 5 In Australia, national unification of child growth monitoring occurred in 2012, when the NHMRC recommended the WHO Child Growth Standards for use in all infants aged 0 to 2 years of age. 6 The current situation in fetal medicine Identifying babies experiencing poor or excessive growth in utero is challenging. Despite ASUM recommending the Campbell Westerway charts in 2000,7 there has been no consensus on fetal growth monitoring, no publication of an ‘Australian standard’, and consequently several charts have been used. For most practitioners, the choice of fetal growth chart is determined either by their institutional protocol, professional society, imaging software program, or the default chart installed by the ultrasound machine manufacturer. However, charts differ greatly not only in the centile thresholds and trajectories, but also the quality of the studies upon which they were based: two comprehensive systematic reviews evaluated the quality of published ultrasound charts for fetal dating with crown‐rump length 8 and fetal growth monitoring. 9 Across the 112 studies identified, there were several important potential sources of methodological bias including: failure to define gestational age accurately; inconsistent population definitions and inclusion and exclusion criteria; lack of image standardisation protocols, and retrospective analysis of images captured for clinical purposes. This resulted in a large amount of variation in centile thresholds when different charts were used: for example, the 10th centile for AC at 36 weeks' gestation ranged from 276 to 292 mm even among the best studies. Open in a separate window Figure 1 Abdominal circumference.