Congenital Microcephaly Case Definitions
Pregnancy Outcomes and Microcephaly Diagnosis
|Diagnosing Microcephaly||Pregnancy Outcome|
|Definite Congenital Microcephaly||
For Live Births: Head circumference (HC) at birth less than the 3rd percentile for gestational age and sex OR if HC at birth is not available, HC less than the 3rd percentile for age and sex within the first 2 weeks of life
For Stillbirths and Elective Terminations: HC at delivery less than the 3rd percentile for gestational age and sex
|Possible Congenital Microcephaly||
For Live Births: If earlier HC is not available, HC less than the 3rd percentile for age and sex beyond 6 weeks of life
For All Pregnancy Outcomes: Microcephaly diagnosed or suspected on prenatal ultrasound in the absence of available postnatal HC measurements
Charts for Head Circumference at Birth
|Gestational Age||Reference Chart for Head Circumference at Birth|
|24–32 Weeks||INTERGROWTH-21st Very Preterm Size at Birth References and tool for calculating centiles for head circumference for infants 24–32 weeks|
|<24 Weeks||INTERGROWTH-21st Fetal Growth Standards|
INTERGROWTH-21st Fetal Growth Standards are based on measurements in utero only. International standards for birth measurements in infants less than 24 weeks gestation are not available. For most elective pregnancy terminations and many stillbirths, accurate postnatal head circumference measurements are not possible.
For a study comparing head circumference measurements in utero to those obtained after birth, see: Melamed N, Yogev Y, Danon D, et al. Sonographic estimation of fetal head circumference: how accurate are we? Ultrasound Obstet Gynecol 2011;37:65-71.
In the setting of Zika virus infection, the head circumference might be disproportionately small in comparison with the length of the infant and not explained by other etiologies. View more information about Zika Virus Infection and Microcephaly.
This video demonstrates how to measure the head circumference of an infant. Measuring head circumference is a primary way to determine the presence of microcephaly, which is important during this Zika outbreak. It is intended to help clinicians evaluate babies born to mothers infected with Zika virus.
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- Page last reviewed: October 20, 2016
- Page last updated: October 20, 2016
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