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Prenatal Diagnosis of Microcephaly

Why is fetal ultrasound recommended?

Fetal ultrasound is generally performed in pregnancies between 18-20 weeks of gestation to assess fetal anatomy as part of routine obstetrical care.  Although microcephaly and intracranial calcifications are typically detected during ultrasounds in the late second and early third trimester of pregnancy, these findings might be detected as early as 18-20 weeks gestation. Zika can cause microcephaly and other severe brain defects, and Zika virus infection during pregnancy has been linked to other adverse pregnancy and birth outcomes, including fetal loss and eye abnormalities. Hence, additional ultrasounds might provide an opportunity to identify findings consistent with congenital Zika virus infection and assist in determining clinical management.

Is ultrasound safe in pregnancy?

Ultrasound is performed during pregnancy when medical information is needed. It has been used during pregnancy for many years and has not been associated with adverse maternal, fetal, or neonatal outcomes. Ultrasound operators are trained to use the lowest power for the minimum duration of time to obtain the needed information. There is consensus among various national and international medical organizations (American College of Radiology, American College of Obstetricians and Gynecologists, and the Society of Maternal and Fetal Medicine) that ultrasound is safe for the fetus when used appropriately.

What prenatal ultrasound findings have been observed among infants with confirmed Zika virus infection?

Brain abnormalities reported in infants with laboratory-confirmed congenital Zika virus infection include microcephaly and disrupted brain development. Some infants with possible Zika virus infection have been found to have intracranial calcifications and eye abnormalities. It is not known if Zika virus infection caused any of these abnormalities.

Ultrasound findings that have been associated with Zika virus disease include microcephaly and other severe brain defects. Studies are under way to learn more about the full spectrum of abnormalities (detected prenatally and postnatally) that might be associated with congenital Zika infection.

How is microcephaly diagnosed prenatally?

Microcephaly can be diagnosed during pregnancy with ultrasound. Microcephaly is most easily diagnosed by ultrasound late in the second trimester or early in the third trimester of pregnancy.

How early can microcephaly be diagnosed during pregnancy?

Microcephaly might be detected as early as 18-20 weeks of gestation however, detection by prenatal ultrasound can be challenging at this gestational age due to fetal position and fetal motion artifact. The optimal time to perform ultrasound screening for fetal microcephaly is not known. In the absence of microcephaly, the presence of intracranial calcifications before 22 weeks gestation might suggest a risk for the future development of microcephaly.

How accurately can ultrasound detect microcephaly in the context of a maternal Zika virus infection?

The accuracy of ultrasound to detect microcephaly in the setting of maternal Zika virus is not known and will depend on many factors such as the timing of maternal infection relative to the timing of screening, severity of microcephaly, patient factors (e.g., obesity), gestational age, equipment used, and the expertise of the person performing the ultrasound. Because the absence of congenital microcephaly and intracranial calcifications on ultrasound at one point in pregnancy does not exclude future microcephaly, additional ultrasounds may be considered at the discretion of the healthcare provider. CDC will update guidance for women and their healthcare providers as more information related to Zika virus infection and microcephaly becomes available.

If a prenatal ultrasound demonstrates microcephaly, how well does it predict microcephaly in the infant?

The sensitivity of prenatal ultrasound for detection of microcephaly depends on a range of factors (e.g., timing of screening, severity of microcephaly, patient factors). In a study of congenital microcephaly not caused by Zika virus infection, prenatally diagnosed microcephaly correlated with neonatal microcephaly approximately 57% of the time.

Can fetal MRI be used to detect microcephaly?

Fetal MRI is not a screening tool and should be used only to answer specific questions raised by ultrasound or used in occasional specific high-risk situations. Interpretation of fetal MRI requires specialized expertise and has limited availability in the United States.