Increasing obstetric intervention for fetal growth restriction is shifting birthweight centiles: a retrospective cohort study

Abstract

Objective To assess the impact of increasing obstetric intervention on birthweight centiles.

The pitfalls of using birthweight centile charts to audit care

“…Recalculating the birthweight centiles each decade to account for changes in population characteristics ensures that the charts remain an accurate descriptive representation of the population. However, it was recently reported that the effect of increasing intervention preferentially removing smaller babies from later gestations is likely to be the primary driver for shifting birthweight centiles [3]. Our observations in this study confirm this, illustrating that the proportion of births <3 rd and <10 th centile have decreased over time in each birthweight chart.…”

“…Our observations in this study confirm this, illustrating that the proportion of births <3 rd and <10 th centile have decreased over time in each birthweight chart. That the effect was primarily at term gestations and not preterm gestations suggests that the shifting of centiles is an artefactual effect of increasing intervention, as previously suggested [3], rather than true biological increases in fetal size as a result of changing maternal characteristics.…”

“…Of course, part of the problem is that we are using a postnatal birthweight measure to judge antenatal decisions that use ultrasound-derived estimated fetal weight centiles. Updating population-based birthweight centiles will lead to increasing drift between the birthweight centiles used to audit care and ultrasound-derived centiles used to make clinical decisions [3]. With a centile distribution alone, there will always be a tradeoff between sensitivity and specificity.…”

“…However, increasing intervention for FGR is changing the very definition of small, as defined by population-derived birthweight centiles [3], the "yardstick" by which clinical care is audited and hospital performance is assessed [4][5][6][7]. This has been shown most recently in Victoria, Australia, where improved detection of FGR has reduced the proportion of babies with birthweight <3 rd centile from 3.1% to 1.9% [2].…”

“…Over time, this will cause the very birthweight cutoff that defines the 3 rd centile to increase. In that same Australian population it has been projected that the birthweight defining the 3 rd centile at 40 weeks' gestation will increase by 150 grams over 35 years [3]. This effect occurs because of the detection and earlier delivery of smaller babies, removing them from the population at term and thereby right-shifting the birthweight distribution.…”