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Microcephaly refers to an abnormally small head defined as a head circumference (HC) of 3 standard deviations (SD) or more below the mean for the gestational age [1-4]. Microcephaly can be present at birth or it may develop in the first few years of life [6] . 

Prenatally, Chervenak et al. found fetal microcephaly could not be reliably diagnosed or excluded on the basis of a single ultrasound examination if the HC was between 2 SD and 5 SD below the mean  [2]. The Society for Maternal-Fetal Medicine (SMFM) recommends that a detailed neurosonographic examination be done on fetuses with an HC greater than 2 standard deviations below the mean to look for findings such as intracranial calcifications, cerebellar hypoplasia and other abnormalities. The finding of a sloping forehead should increase the index of suspicion for pathological microcephaly. The SMFM also  advises the diagnosis of pathologic microcephaly is considered certain when the fetal HC is equal to or greater than 5 SD below the mean for the gestational age [4]. A follow up ultrasound in 3 to 4 weeks is recommended for fetuses with an HC more than 2 SD below the mean.

Pathological microcephaly is usually associated with abnormal neurological findings , mental retardation, and sometimes seizures.  Causes of microcephaly include , but are not limited to, a chromosomal abnormality (trisomy or deletion),  a gene defect such as Smith-Lemli-Opitz, an infection such as rubella, cytomegalovirus (CMV),  toxoplasmosis, or possibly Zika virus, intrauterine fetal stroke, exposure to high doses of radiation, exposure to drugs such as alcohol, aminopterin ,or hydantoin, or a maternal condition such as phenylketonuria (PKU).

Microcephaly occurs in about 2 to 12 babies per 10,000 live births in the Unites States [5].

Head Circumference with Standard Deviations (pdf)

Head Circumference with Standard Deviations (jpg)


1. Chervenak FA, Jeanty P, Cantraine F, Chitkara U, Venus I, Berkowitz RL, et al. The diagnosis
of fetal microcephaly. Am J Obstet Gynecol. 1984;149:512-7 PMID:6742021
2. Chervenak FA, et. al., A prospective study of the accuracy of ultrasound in predicting fetal microcephaly.
Obstet Gynecol. 1987 Jun;69(6):908-10. PMID:3554067
3. Leibovitz Z, et al. Microcephaly at birth - the accuracy of three references for fetal head circumference. How can we improve prediction?  Ultrasound Obstet Gynecol. 2015 Oct 29. doi: 10.1002/uog.15801 PMID:26511765
4. Ultrasound screening for fetal microcephaly following Zika virus exposure. Society for Maternal-Fetal Medicine (SMFM) Publications Committee. Am J Obstet Gynecol. 2016 Feb 19. pii: S0002-9378(16)00343-4. doi: 10.1016/j.ajog.2016.02.043.
5. National Birth Defects Prevention Network. Major birth defects data from population-based birth defects surveillance programs in the United States, 2006-2010. Birth Defects Research (Part A): Clinical and Molecular Teratology. 2013;97:S1-S172.
6. "NINDS Microcephaly Information Page". NINDS. March 14, 2016. Retrieved April 3 2016.
7.Brasil P,, K.Zika Virus Infection in Pregnant Women in Rio de Janeiro - Preliminary Report.
N Engl J Med. 2016 Mar 4. PMID:26943629

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