Prenatal Care

Diagnosing Microcephaly

Microcephaly is defined as a head circumference measurement that is smaller than a certain value for fetuses or infants of the same age and sex. This measurement value for microcephaly is usually less than 2 standard deviations (SDs) below the average. The measurement value also may be designated as less than the 3rd percentile.

Microcephaly may 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. 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. 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. In addition, fetal abnormalities, including microcephaly, may not be apparent on the fetal ultrasound, but may be apparent at delivery or develop postnatally.

Clinical Management of Pregnant Women with Possible Zika Virus Infection

For pregnant women with confirmed or possible Zika virus infection, serial fetal ultrasounds (every 3–4 weeks) should be considered to assess fetal anatomy, particularly fetal neuroanatomy, and to monitor growth closely. Given the length of time for the detection of prenatal microcephaly, prenatal ultrasounds should include a detailed fetal anatomy, particularly neuroimaging, to detect brain or structural abnormalities that might occur before microcephaly. For additional clinical management recommendations during pregnancy, refer to CDC’s updated interim guidance.

Fetal Ultrasound

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 birth defects might be detected as early as 18-20 weeks gestation. A recent study of 17 pregnancies with laboratory confirmed Zika virus infection and adverse fetal outcomes reported a median of 18 weeks from symptom onset to prenatal diagnosis of microcephaly. This finding is consistent with other reports about prenatal diagnosis of microcephaly; among 37 pregnancies with confirmed or suspected Zika virus infection, a median of 21 weeks (range = 3–29 weeks) from maternal symptom onset to prenatal diagnosis of microcephaly was observed. Given the length of time for the detection of prenatal microcephaly, prenatal ultrasounds should include a detailed fetal anatomy, particularly neuroimaging, to detect other brain or structural abnormalities that might occur before microcephaly.

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 birth defects consistent with congenital Zika virus infection and assist in determining clinical management.

Safety

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.

Accuracy

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.

Sensitivity

The sensitivity of prenatal ultrasound for detection of microcephaly and brain abnormalities 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.

Fetal MRI

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.

Amniocentesis

Amniocentesis is a medical procedure in which a small amount of amniotic fluid is removed from the sac surrounding the fetus for testing. Consideration of amniocentesis should be individualized based on the patient’s clinical circumstance. Amniocentesis has been used in the evaluation of other congenital infections and may be considered in the evaluation of potential Zika virus infection. Healthcare providers should discuss the risks and benefits of amniocentesis with their patients.

Amniocentesis is not recommended until after 15 weeks of gestation. Amniocentesis performed at ≥15 weeks of gestation is associated with lower rates of complications than when performed at earlier gestational ages (≤14 weeks of gestation). However, the optimal time to perform amniocentesis to diagnose congenital Zika virus infection is not known and should be individualized according to the patient’s clinical circumstances. Referral to a maternal-fetal medicine specialist may be warranted. Healthcare providers should discuss the risks and benefits of amniocentesis with their patients.

A positive Zika virus RNA NAT result from amniotic fluid might indicate fetal infection. This information would be useful for pregnant women and their healthcare providers to assist in determining clinical management (e.g., antepartum testing, scheduling serial ultrasounds, delivery planning). Although a negative Zika virus RNA NAT result from amniotic fluid does not exclude congenital Zika virus infection, it may prompt a work-up for other causes of microcephaly (e.g., other infections, genetic disorders).

We do not know:

• The optimal time to perform amniocentesis to diagnose congenital Zika virus infection.
• How sensitive or specific tests of amniotic fluid are for congenital Zika virus infection.
• If a positive result is predictive of a subsequent fetal abnormality.
• If a positive result is predictive, what proportion of infants born after infection will have abnormalities.