The pitfalls of using birthweight centile charts to audit care

Timely delivery of fetal growth restriction (FGR) is important in reducing stillbirth. However, targeted earlier delivery of FGR preferentially removes smaller babies from later gestations, thereby right-shifting the distribution of birthweights at term. This artificially increases the birthweight cutoffs defining the lower centiles and redefines normally grown babies as small by population-based birthweight centiles. Our objective was to compare updated Australian national population-based birthweight centile charts over time with the prescriptive INTERGROWTH-21st standard.

A retrospective descriptive study of all singleton births ≥34 weeks' gestation in Victoria, Australia in five two-year epochs: 1983-84, 1993-94, 2003-04, 2013-14, and 2016-17. The birthweight cutoffs defining the 3rd and 10th centile from three Australian national population-based birthweight centile charts, for births in 1991-1994, in 1998-2007, and 2004-2013 respectively, were applied to each epoch to calculate the proportion of babies with birthweight <3rd and <10th centile. The same analysis was done using the INTERGROWTH-21st birthweight standard. To assess change over gestation, proportions were also calculated at preterm, early term and late term gestations.

From 1983-84 to 2016-17, the proportion of babies with birthweight <3rd fell across all birthweight centile charts, from 3.1% to 1.7% using the oldest Australian chart, from 3.9% to 1.9% using the second oldest Australian chart, from 4.3% to 2.2% using the most recent Australian chart, and from 2.0% to 0.9% using the INTERGROWTH-21st standard. A similar effect was evident for the <10th centile. The effect was most obvious at term gestations. Updating the Australian population birthweight chart progressively right-shifted the birthweight distribution, changing the definition of small over time. The birthweight distribution of INTERGROWTH-21st was left-shifted compared to the Australian charts.

Locally-derived population-based birthweight centiles are better for clinical audit of care but should not be updated. Prescriptive birthweight standards are less useful in defining 'small' due to their significant left-shift.