Development of a birthweight standard and comparison with currently used standards. What is a 10th centile?

Introduction: Fetal growth charts are often used in clinical practice. It is important to understand the usefulness and the pitfalls associated with these tools. Without validation, it is difficult to ascertain if the cutoffs we intend are the ones we actually select. We developed a national standard for birthweight (BW) and compared it with other published reference values.

Study design: Multicenter retrospective study. We collected data on live births, including first trimester ultrasound and pathology, from 23 to 42 weeks' gestational age (GA). We used a variation of the lambda (λ), mu (μ), and sigma (σ) method (LMS) to construct and smooth predicted centiles. GA data was plotted and modeled in days from 24 to 42 weeks. Resulting centiles were validated and compared with other published and widely used reference values. Data from both BW and estimated fetal weight was used to validate the model.

Results: Data on 661,338 births were collected from 22 institutions, including 71,515 cases with first trimester ultrasound. We excluded preterm cesarean section from analysis, because of a significant bias (up to 18%) on BW and used exclusively first trimester ultrasound dates from 34 to 42 weeks. The standard compares favorably with tables currently in use, both ultrasound and birthweight based.

Conclusion: The use of first trimester ultrasound limits variability by minimizing some random error sources, such as data introduction and GA errors, while allowing better precision (GA in days). This results in a narrower range in the extreme centiles than other charts. Validation with estimates of fetal weight are sound in second and early third trimester fetuses, because that will be a "real world" usage of this standard. While there are similarities between our series and some international/foreign growth charts, other are unfit to characterize our population. This reinforces the need for validation of standards, and sound methodological practices when doing so.