Volume 11 Issue 1 (February) - Australasian Society for Ultrasound ...

DIAGNOSTIC ULTRASOUND
ASUM Ultrasound Bulletin February 2008; 11 (1): 12–18
Australian charts for assessing fetal growth: ­
a review
Lisa Hui
Division of Maternal Fetal Medicine, Royal Hospital for Women, Barker St, Randwick NSW 2031, Australia.
Correspondence to Lisa Hui. Email lisahui77@gmail.com
Introduction
Monitoring fetal growth is one of the basic goals of antenatal
care, for low-risk and high-risk women alike. While the
‘large for gestational age’ fetus is at increased risk of perinatal
complications, the burden of adverse outcomes is overwhelmingly
carried by the pathologically small fetus. Fetal
growth restriction is accompanied by dramatically increased
perinatal mortality, with mortality being eight times higher
when weight is below the 10th centile and nearly 20 times
higher when weight is below the 3rd centile 1 . In addition to
increased mortality, both short term 2,3 and long term morbidity
4,5 are increased in infants affected by intrauterine growth
restriction.
The Ultrasound Bulletin has previously published a
review on the accuracy of sonographic estimation of fetal
weight that discusses the sources of error in determining
whether or not a given fetal size is ‘normal’ 6 . This paper
seeks to further examine the centile charts in current use
in Australia and to describe their derivation. The multitude
of biometry and birthweight charts can cause confusion
if a fetus is compared against different standards during
pregnancy and this has the potential to cause inappropriate
management decisions. Finally, the rising interest in individualised
birthweight standards and their role in developing
intrauterine growth curves will also be covered.
Definitions of SGA/FGR
Various growth percentile thresholds for defining ‘small for
gestational age’ (SGA) babies have been used, but the most
commonly accepted standard is the 10th centile 7 for estimated
fetal weight (EFW) or abdominal circumference (AC).
However, it is important to remember that babies below the
10th centile are a heterogeneous group comprising of constitutionally
small normal babies and those with true growth
restriction with the accompanying increase in morbidity and
mortality. Identifying those SGA babies at increased risk of
true fetal growth restriction (FGR) involves incorporating
other measures of fetal well-being such as amniotic fluid
volume, fetal arterial and venous Doppler measurements and
cardiotography 8,9 .
Ultrasound estimation of fetal size
There are several discrete steps in the process of ultrasound
estimation of fetal growth. Each step is subject to error. An
acceptable margin of error for the final EFW is generally
accepted to be ± 15%. These steps are:
1 Accurate assessment of dates;
2 Fetal measurements: two-dimensional measurement of
biometry;
3 Mathematical calculation of an estimated fetal weight;
4 Charting fetal size – EFW and/or biometry – against
population standards for gestation and determining the
corresponding percentile band; and
5 Comparing serial measurements if available, to determine
whether growth velocity is being maintained along
the appropriate centile curve.
Determining intrauterine weight percentile
The ideal chart should be created from a representative sample
of the local population. These growth charts can be constructed
from birthweight data for preterm and term infants,
or from standards derived from ultrasound measurements of
fetuses. A third method has been recently investigated using
customised term birthweights to develop intrauterine growth
curves.
Australian birthweight charts
Until the late 1990s, the most commonly used standards
of growth in Australia were derived from hospital-based
studies of infants born in Melbourne. The first publications
were produced by Kitchen in 1968 10 and Betheras in
1969 11 . Explicit in the publication of Betheras’ charts was
the caveat that the use of birthweights to define normal
growth at preterm gestations contains an inherent fault: ‘the
obstetrical complication which may have precipitated the
premature termination of pregnancy may have affected fetal
growth.’ These early charts are disadvantaged by the less
accurate dating methods, with early ultrasound dating being
uncommon. Betheras’ chart included women if there was
‘reasonable certainty’ that the EDC was correct, and if the
infant had a weight and head circumference (HC) measured
‘shortly after birth’. These early charts also have very small
numbers of infants at extremely preterm gestations, and in
Betheras’ case, only commence from 28 weeks gestation.
There have been significant changes in ethnic composition
and socioeconomic factors in the Australian population
since the publication of Betheras’ chart, which was based
almost entirely on women of Anglo-Saxon origin. Kitchen
updated early data from 1968 and in 1983 produced revised
intrauterine growth curves from livebirth data 12 . Birthweight
curves from 24 to 42 weeks were produced from a combination
of data from live births at the Royal Women’s Hospital
in Melbourne in 1979 and from previous publications on
those born