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Measurement error, microcephaly prevalence and implications for Zika: an analysis of Uruguay perinatal data
  1. Emily W Harville1,
  2. Pierre M Buekens1,
  3. Maria Luisa Cafferata2,
  4. Suzanne Gilboa3,
  5. Giselle Tomasso4,
  6. Van Tong3
  1. 1Department of Epidemiology, Tulane School of Public Health and Tropical Medicine, Tulane University, New Orleans, Louisiana, USA
  2. 2Institute for Clinical Effectiveness and Health Policy, Buenos Aires, Argentina
  3. 3Division of Congenital and Developmental Disorders, National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, Atlanta, Georgia, USA
  4. 4Clinical and Epidemiological Research Unit, Montevideo, Uruguay
  1. Correspondence to Dr Emily W Harville, Tulane University, New Orleans, LA 70112, USA; eharvill{at}


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.

  • microcephaly
  • bias
  • head circumference
  • gestational age
  • Zika virus infection
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  • Contributors EWH performed data analysis and drafted the manuscript. PMB conceived the paper, assisted with study design and secured funding. MLC and SG assisted with study design and interpretation. GT facilitated data access and assisted with study design and interpretation. VT assisted with study design, interpretation and securing of funding. All authors contributed to manuscript development and edited for critical content, and have approved the final version.

  • Funding This study was funded by Vysnova Partners (SC-2017-3034-TU).

  • Disclaimer The findings and conclusions of this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; internally peer reviewed.

  • Data availability statement Data may be obtained from a third party and are not publicly available.

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