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. 2020 May;105(5):428-432.
doi: 10.1136/archdischild-2019-317678. Epub 2019 Dec 13.

Measurement error, microcephaly prevalence and implications for Zika: an analysis of Uruguay perinatal data

Emily W Harville  1 , Pierre M Buekens  2 , Maria Luisa Cafferata  3 , Suzanne Gilboa  4 , Giselle Tomasso  5 , Van Tong  4
Affiliations

Measurement error, microcephaly prevalence and implications for Zika: an analysis of Uruguay perinatal data

Emily W Harville et al. Arch Dis Child. 2020 May.

Abstract

Background and objective: The Zika virus outbreak has drawn attention to microcephaly, whose definition is based on head circumference measuring below a percentile or number of SDs below the mean. The objective of this analysis was to assess how differences in measurement precision might affect prevalence and trends of microcephaly.

Methods: Data from all births in Uruguay during 2010-2015 were obtained from the Perinatal Information System. The prevalence of births with microcephaly was calculated based on head circumference measurement at birth applying the INTERGROWTH-21st standards for sex and gestational age, and compared by method of ascertaining gestational age.

Results: Rounding and digit preference was observed: 74% of head circumference measurements were reported as a whole centimetre value. The prevalence of births varied substantially by the criterion used to define microcephaly (<3 SD, <2 SD, <3rd percentile for gestational age) and could be halved or doubled based on adding or subtracting a half-centimetre from all reported head circumference measurements. If 4 days were added to gestational age calculations, rather than using completed gestational weeks (without days) for gestational age reporting, the prevalence was 1.7-2 times higher.

Discussion: Rounding in measurement of head circumference and reporting preferences of gestational age may have contributed to a lower prevalence of microcephaly than expected in this population. Differences in head circumference measurement protocols and gestational age dating have the potential to affect the prevalence of babies reported with microcephaly, and this limitation should be acknowledged when interpreting head circumference data collected for surveillance.

Keywords: Zika virus infection; bias; gestational age; head circumference; microcephaly.

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Conflict of interest statement

Competing interests: None declared.

Figures

Figure 1
Figure 1
Head circumference with normal and kernel smoothers, implausible values removed. Top, bin width=1, bottom, bin width=1.1.
Figure 2
Figure 2
Mode of delivery and prevalence of microcephaly, by gestational age (GA) (>28 weeks), implausible head circumference data removed (>5 SD from the mean, n=253 452).
Figure 3
Figure 3
Methods of ascertaining gestational age and prevalence of microcephaly, by gestational age, implausible data removed (limited to HC±5 SD (cm)) and GA >28 weeks. GA, gestational age; LMP, last menstrual period.
See this image and copyright information in PMC

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