Conclusions

Using growth standards that rely on a common centile cutoff for every day of a given gestational week, an infant is significantly more likely to be considered small if born earlier in the gestational week, and significantly less likely if born at the end of the week. This leads to an artefactual distortion of the SGA cohorts, and unequal distribution of relative risk of stillbirth by days of the week. This anomaly can be overcome by the use of charts that provide a centile cutoff for each day of the week, producing uniform SGA classification across the week, and correcting the distorted risk of stillbirth. We therefore recommend use of growth standards that give an individual centile cutoff for each day of the week. If this is not practical, we urge clinicians to be cognisant of the timing of assessment within the week when making management decisions.

Acknowledgements

We are grateful to CCOPMM for providing access to the data used for this project and for the assistance of the staff at Safer Care Victoria. The conclusions, findings, opinions and views or recommendations expressed in this paper are strictly those of the author(s). They do not necessarily reflect those of CCOPMM. NP receives a National Health and Medical Research Council Postgraduate research scholarship, and a top-up scholarship from the national Stillbirth Centre of Research Excellence. AL receives salary support from an NHMRC Ideas grant and ST receives salary support from an NHMRC Practitioner’s Fellowship. The funding sources were not involved in the research.