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Prenatal Care

During pregnancy, healthcare providers can monitor the fetus for signs of congenital Zika virus infection. This page describes what is known about the use of ultrasound and amniocentesis in the clinical management of pregnant women with possible Zika virus infection.

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 movement. 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 updated clinical management recommendations during pregnancy, refer to CDC’s updated interim guidance.

Fetal Ultrasound

Routine ultrasound screening for fetal abnormalities is a component of prenatal care in the United States. Comprehensive ultrasound examination to evaluate fetal anatomy is recommended for all women at 18–22 weeks’ gestation.
The optimal timing of ultrasound among pregnant women with possible maternal Zika virus exposure remains unknown. Abnormalities have been detected anywhere from 2 to 29 weeks after symptom onset, and is dependent on when the infection occurred. Insufficient data are available to define the optimal timing between Zika virus exposure and initial sonographic screening. In published case reports, brain abnormalities associated with congenital Zika syndrome have been identified by ultrasound in the second and third trimesters. CDC previously recommended serial ultrasounds every 3-4 weeks for women exposed during pregnancy with laboratory evidence of Zika virus infection. This recommendation was adopted from existing guidelines for fetal growth monitoring for other maternal conditions. In the absence of data to guide timing of ultrasound to detect abnormalities related to Zika virus infection, healthcare providers may consider extending the time interval between ultrasounds in accordance with clinical judgement and patient preferences.

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.

In a study of congenital microcephaly not caused by Zika virus infection, prenatally diagnosed microcephaly correlated with neonatal microcephaly approximately 57% of the time. Recent data suggest that several abnormalities that have been identified during prenatal ultrasound (e.g., microcephaly, ventriculomegaly, or abnormalities of the corpus callosum) in the context of maternal Zika virus exposure correlate with reported structural abnormalities in infants at birth.

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. The role of amniocentesis for the detection of congenital Zika virus infection is unknown. 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.

Amniocentesis is not recommended until after 15 weeks’ gestation. Amniocentesis performed at ≥15 weeks’ 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.

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). There is a dearth of data about the correlation between positive Zika test results in amniotic fluid and clinical phenotype or confirmatory infant laboratory testing. In addition, although Zika virus RNA has been detected in amniotic fluid specimens; serial amniocenteses have demonstrated that Zika virus RNA might only be present transiently. Thus, a negative Zika virus RNA NAT result from amniotic fluid does not exclude congenital Zika virus infection. Evaluation for other causes of microcephaly (e.g., other infections, genetic disorders) should be considered for infants with birth defects.

We do not know:

  • The optimal time to perform amniocentesis to diagnose congenital Zika virus infection.
  • The accuracy of the tests on amniotic fluid for congenital Zika virus infection.
  • If a positive result is predictive of a fetal/infant infection.
  • If a positive result is predictive, what proportion of infants with infection will have abnormalities.
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