Original Research
Obstetrics
From population reference to national standard: new and improved birthweight charts

https://doi.org/10.1016/j.ajog.2018.12.023Get rights and content

Background

Antenatal detection of intrauterine growth restriction remains a major obstetrical challenge, with the majority of cases not detected before birth. In these infants with undetected intrauterine growth restriction, the diagnosis must be made after birth. Clinicians use birthweight charts to identify infants as small-for-gestational-age if their birthweights are below a predefined threshold for gestational age. The choice of birthweight chart strongly affects the classification of small-for-gestational-age infants and has an impact on both research findings and clinical practice. Despite extensive literature on pathological risk factors associated with small-for-gestational-age, controversy exists regarding the exclusion of affected infants from a reference population.

Objective

This study aims to identify pathological risk factors for abnormal fetal growth, to quantify their effects, and to use these findings to calculate prescriptive birthweight charts for the Dutch population.

Materials and Methods

We performed a retrospective cross-sectional study, using routinely collected data of 2,712,301 infants born in The Netherlands between 2000 and 2014. Risk factors for abnormal fetal growth were identified and categorized in 7 groups: multiple gestation, hypertensive disorders, diabetes, other pre-existing maternal medical conditions, maternal substance (ab)use, medical conditions related to the pregnancy, and congenital malformations. The effects of these risk factors on mean birthweight were assessed using linear regression. Prescriptive birthweight charts were derived from live-born singleton infants, born to ostensibly healthy mothers after uncomplicated pregnancies and spontaneous onset of labor. The Box-Cox-t distribution was used to model birthweight and to calculate sex-specific percentiles. The new charts were compared to various existing birthweight and fetal-weight charts.

Results

We excluded 111,621 infants because of missing data on birthweight, gestational age or sex, stillbirth, or a gestational age not between 23 and 42 weeks. Of the 2,599,640 potentially eligible infants, 969,552 (37.3%) had 1 or more risk factors for abnormal fetal growth and were subsequently excluded. Large absolute differences were observed between the mean birthweights of infants with and without these risk factors, with different patterns for term and preterm infants. The final low-risk population consisted of 1,629,776 live-born singleton infants (50.9% male), from which sex-specific percentiles were calculated. Median and 10th percentiles closely approximated fetal-weight charts but consistently exceeded existing birthweight charts.

Conclusion

Excluding risk factors that cause lower birthweights results in prescriptive birthweight charts that are more akin to fetal-weight charts, enabling proper discrimination between normal and abnormal birthweight. This proof of concept can be applied to other populations.

Section snippets

Procedures

Data were extracted from the Dutch perinatal database, which is a linked database of medical registries from the 4 professional organizations that provide perinatal care in The Netherlands: the NVOG, KNOV, NVK and the National Association of General Practitioners (LHV).11 Registration of obstetric data starts at the first antenatal visit; the neonatal registry contains data only on neonates who were hospitalized following delivery.12 In over 95% of pregnancies, gestational age is certain,

Results

Between 2000 and 2014, a total of 2.7 million infants were born in The Netherlands. We excluded 112,661 infants (4.2%) because of missing data, fetal death, or gestational age not between 23 and 42 weeks (Figure 1). To obtain a low-risk population, another 969,552 infants (37.3%) were excluded because of the presence of 1 or more risk factors.

Figure 2 shows the differences in mean birthweight between infants with risk factors vs infants without. From 31 weeks’ gestation onward, twins and

Comment

This article describes the development of prescriptive birthweight charts, derived from a large population of Dutch infants without risk factors for SGA or excessive fetal growth.

Numerous authors in many countries have put a great deal of thought into the desired properties of birthweight charts. Despite extensive literature on the subject, there is little evidence to support a particular approach. Sources of controversy include the choice of infants who should be included in a population and

Acknowledgments

This research was supported by the Isala Science and Innovation Fund and Nutricia Early Life Nutrition. The sponsors had no role in the design and conduct of the study, collection, management, analysis, and interpretation of the data, or in the preparation, review, approval of the manuscript, or decision to submit the manuscript for publication.

We thank our collaborators from the Working Group Reference Charts who provided insight and expertise that greatly assisted the research:

Dr J. van

References (31)

  • C. Ferdynus et al.

    Comparison of the ability of alternative birthweight and fetal weight standards to identify preterm newborns at increased risk of perinatal death

    Br J Obstet Gynaecol

    (2013)
  • L. Hoftiezer et al.

    Prescriptive birthweight charts can improve the prediction of adverse outcomes in very preterm infants who are small for gestational age

    Acta Paediatr

    (2018)
  • A.L. Tarca et al.

    A new customized fetal growth standard for African American women: the PRB/NICHD Detroit study

    Am J Obstet Gynecol

    (2018)
  • Perinatale zorg in Nederland 2016

    (2018)
  • A.A. Vos et al.

    Differences in perinatal morbidity and mortality on the neighbourhood level in Dutch municipalities: a population based cohort study

    BMC Pregnancy Childbirth

    (2015)
  • Cited by (129)

    • Attenuated cognitive functioning decades after preeclampsia

      2023, American Journal of Obstetrics and Gynecology
    View all citing articles on Scopus

    Dr Hukkelhoven is currently at Wageningen University and Research, Wageningen Data Competence Center, Wageningen, The Netherlands.

    The authors report no conflict of interest.

    This work was supported by unrestricted research grants from Nutricia Early Life Nutrition and the Isala Innovation and Science Fund.

    Cite this article as: Hoftiezer L, Hof MHP, Dijs-Elsinga J, et al. From population reference to national standard: new and improved birthweight charts. Am J Obstet Gynecol 2019;220:383.e1-17.

    View full text