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

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Abstract

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.

Introduction

Obstetricians and neonatologists worldwide are familiar with the concept of fetal growth charts, arguably introduced by Lubchenco et al. in 1963 [1], in which birthweight is plotted as a function of gestational age (GA). As growth centiles correlate with fetal and neonatal outcomes, these are often used to produce clinical judgements. While several limitations and pitfalls are apparent, these are readily available tools, easy to understand and widely adopted [2], [3]. Given the large number of such charts published over the years, it is up to the clinician to choose one to which compare its population of fetuses/newborns. Such choice and its implications are not always well understood [4] and even professional societies sometimes do not agree on recommendations based on available evidence [5].

Whatever the tool, validation is of paramount importance, to ensure that, when the clinician selects the 10th centile, he is indeed referring to 10% of his intended population (e.g., a healthy local woman). We aim to describe how a population standard relates to others, and if differences are clinically significant, given the regional, ethnical and geographical differences previously identified [6], [7], [8]. We describe a birthweight (BW) standard for Portugal, from 24 to 42 weeks and compare it with other commonly used charts.

Section snippets

Participants

Singleton neonates from gestational age (GA) 168–300 days (23–42 completed weeks), live born between January 2004 and June 2014. Mothers with medical pathology were excluded from analysis (hypertensive diseases and diabetes, gestational or otherwise; autoimmune diseases; epilepsy; chronic medications). Newborn malformations, including chromosomal abnormalities, and hemolytic diseases were also excluded.

Study design

Cross sectional study. We identified and sought institutional authorizations for data

Results

Twenty-seven maternity hospitals had more than 1500 births per year (roughly 70% of maternities and 90% of births in the country), of which 24 authorized data collection. Computerized records were retrieved from 22 of these institutions. In the two remaining institutions, it was not possible to retrieve the required minimum set of data (Fig. 1).

Data was not available throughout the whole time frame in some institutions. Ultrasound data was available in 18 institutions. A total of 661,338 live

Discussion

To our knowledge, the number of first trimester ultrasound dated pregnancies is one of the largest published to date in a growth chart, especially considering the use of ultrasound databases, which permitted GA in days from 34 weeks onward. This precision led to a model with more information in the temporal relation between GA and BW. The multiple sources of data may be an issue, but the few database providers, standardized queries and the easily identifiable variables, eased the process of

Conclusion

A birthweight standard is presented and validated. We describe the differences to other charts in use in our country and elsewhere, which can be very large, and, thus, lead to misclassification of the intended cutoffs. Specifically, the Lubchenco [1] table is unfit for use, and, while Yudkin’s provides a comparable 50th centile, its larger range compromises the correct identification of extreme centiles.

The validation data provided a “real world” application scenario, with a novel approach to

Acknowledgments

The authors would like to thank Federação Portuguesa das Sociedades de Obstetrícia e Ginecologia (Portuguese Federation of Obstetrics and Gynecology Societies) for its financial support. We also thank the collaboration of all involved maternities. The following people were especially helpful in the data retrieval process: Tiago Costa, Eliana Sousa, António Lourenço, Rui Ribeiro, Antónia Santos, Rosa Monteiro, Paulo Brás, Abel Amaro, Paulo Moura, Natália Macedo, Jorge Branco, Ricardo Fontes,

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