WHAT IS IT? 

• Microcephaly means “small head”, and can be used interchangeably in some instances with microencephaly, “small brain”
  • Consider abnormal development (primary microcephaly) if detected prior to 32 weeks gestation, as opposed to a degenerative condition in which normal head size becomes smaller (secondary microcephaly)

SYNOPSIS:

Microcephaly has an incidence of 2 to 12 in 10,000 births in the USA and can be diagnosed prenatally via ultrasound (in second or early third trimester) or postnatally via measurement of head circumference (HC).  Microcephaly has been linked to developmental delay, seizures, as well as feeding, vision and hearing problems.  Prognosis depends on the severity of the microcephaly and whether it is associated with other anomalies.

KEY POINTS:

Definitions

SMFM – Fetal Microcephaly

• HC ≥ 3 SD below the mean for gestational age is the recommended definition
• HC ≥ 5 SD below the mean is a certain diagnosis

CDC – Postnatal Congenital Microcephaly

• Definite Congenital Microcephaly
  • HC at birth < 3rd percentile for gestational age and sex
  • If HC not available at birth, HC < 3rd percentile for age and sex within the first 2 weeks of life
  • For Stillbirths and elective terminations, CDC definition based on HC at delivery < 3rd percentile for gestational age and sex
• Possible Congenital Microcephaly
  • For Live Births: If earlier HC is not available, HC < 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

American Academy of Neurology and Child Neurology Society

• At birth
  • HC >2 SD below the mean for age and gender
  • Severe would be considered >3 SD below the mean

Management

• If fetal HC by ultrasound is > 2 SD below the mean for gestational age, a careful evaluation of the fetal intracranial anatomy is indicated
  • If the intracranial anatomy is normal, consider follow-up in 3-4 weeks
• When evaluating the finding of fetal microcephaly, assess for the following
  • Infectious exposures including
    • Zika virus
    • Maternal syphilis, herpes, HIV, rubella, toxoplasmosis, cytomegalovirus
  • Alcohol consumption, exposure to certain medications and/or smoking during pregnancy
  • Genetic conditions, including aneuploidy or single gene disorders
• Many ultrasound packages report HC percentiles and not standard deviations (SD) and often the lowest reported HC measurement is <5th percentile
  • In such cases, SMFM suggests using head circumference (in millimeters) to determine SD below the mean
  • SMFM provides a table (see ‘Learn More – Primary Sources’ below) to determine the number of SD below the mean as a function of gestational age
• Role of MRI: The SOGC Guideline states (III-A evidence)

Once fetal microcephaly is identified by ultrasound, fetal magnetic resonance imaging, when available and if potential findings are likely to alter pregnancy management, may be considered.

Fetal magnetic resonance imaging images should be reviewed by a radiologist with expertise in fetal magnetic resonance imaging.

• Consider referral to a Fetal Maternal Medicine Specialist
• Consider referral to Genetic Counseling, especially if microcephaly is associated with other findings
• Consider referral to Pediatric Neurologist for review of short and long term potential outcomes for the newborn and child with microcephaly

Image credit: Centers for Disease Control and Prevention, National Center on Birth Defects and Developmental Disabilities

Learn More – Primary Sources:

Microcephaly: An Epidemiologic Analysis 

CDC: Congenital Anomalies of the Nervous System: Microcephaly 

Prenatal gender-customized head circumference nomograms result in reclassification of microcephaly and macrocephaly

SMFM: Ultrasound screening for fetal microcephaly following Zika virus exposure

Practice Parameter: Evaluation of the child with microcephaly (an evidence-based review) 

SOGC Guideline 380: Investigation and Management of Prenatally Identified Microcephaly

CDC: Zika Virus For Healthcare Providers

Locate a Genetic Counselor or Genetics services:

Genetic Services Locator-ACMG

Genetic Services Locator-NSGC

Genetic Services Locator-CAGC

Locate a Maternal Fetal Medicine Specialist:

Maternal-Fetal Medicine Specialist Locator-SMFM

Related ObG Topics:

New CDC Interim Guidance on Infants with Possible Zika & Perinatal Review (Oct 2017)

CDC Guidance on Zika and Pregnancy

Zika virus: transmission, symptoms and management

CDC Guidance for Non-Pregnant Patients and Available Diagnostic Tests for Zika Virus