The Centers for Disease Control and Prevention has issued new guidance for pregnant women and their physicians as concern grows over the spread of Zika virus in the Americas and a possible association between infection during pregnancy and severe birth defects.

The guidance, released this week, tells healthcare providers to ask pregnant patients about recent travel to areas where Zika virus is being transmitted. Pregnant women who have traveled to affected areas and developed symptoms such as fever, rash, joint pain or red eyes, or whose ultrasounds show certain fetal abnormalities, should be evaluated for possible infection in coordination with state or local health departments.

The CDC is also recommending that pregnant women consider postponing travel to areas with ongoing Zika transmission. The agency’s recommendations reflect an unusual level of caution around a virus that typically causes mild illness in adults but has raised urgent questions because of reports of microcephaly among newborns in Brazil.

A Virus With New Implications for Pregnancy

Zika is a mosquito-borne flavivirus transmitted primarily by Aedes aegypti mosquitoes, which also spread dengue and chikungunya. Most people infected with Zika do not develop symptoms, and when illness occurs it is usually mild, with fever, rash, joint pain and conjunctivitis lasting several days to a week.

What has changed the public-health significance of the virus is the possible connection to pregnancy. Brazilian health authorities have reported a sharp increase in infants born with microcephaly, a condition in which a baby’s head and brain are smaller than expected. Investigators are studying whether maternal Zika infection is responsible for some of those cases.

The CDC guidance, posted as an early release January 19 and published in the agency’s Morbidity and Mortality Weekly Report, emphasizes that the relationship is still under investigation. The agency says the full spectrum of outcomes associated with Zika infection during pregnancy is not yet known.

That uncertainty is driving conservative recommendations because pregnant women and their physicians have limited tools if exposure occurs. There is no vaccine to prevent Zika infection and no specific antiviral treatment for the disease.

Travel History Becomes Part of Prenatal Care

Under the new guidance, healthcare providers should ask every pregnant woman about recent travel. Women who traveled to an area with ongoing transmission and developed symptoms consistent with Zika disease during travel or within two weeks afterward should be tested in consultation with public-health officials.

Testing may also be considered when prenatal ultrasound identifies fetal microcephaly or intracranial calcifications in a woman with relevant travel history. The CDC notes that laboratory diagnosis can be complicated because antibodies against Zika may cross-react with related viruses such as dengue.

For pregnant women with laboratory evidence of infection, the agency recommends considering serial ultrasound examinations to monitor fetal growth and anatomy. Referral to a maternal-fetal medicine specialist or an infectious-disease physician with pregnancy expertise may also be appropriate.

The guidance does not recommend routine testing for pregnant women who have not traveled to areas with ongoing Zika transmission. That distinction reflects both the current epidemiology and the limited availability of specialized testing.

CDC Expands Travel Precautions

The pregnancy guidance follows a CDC travel alert advising pregnant women to consider postponing travel to countries and territories experiencing Zika transmission. The list includes several destinations in the Caribbean and Central and South America and is expected to change as surveillance identifies new areas of spread.

For women who must travel, the agency recommends strict mosquito precautions. Those include using insect repellent, wearing long sleeves and pants, staying in screened or air-conditioned accommodations and reducing exposure during periods when Aedes mosquitoes are active.

Unlike some mosquito species that feed mainly at dawn or dusk, Aedes aegypti can bite during the day, making continuous prevention important. The mosquito is established in tropical and subtropical regions and is also found in portions of the southern United States.

The presence of the mosquito in parts of this country does not mean widespread domestic Zika transmission is occurring. Most U.S. cases identified to date have been associated with travel. Public-health officials are nevertheless monitoring the situation closely because an infected traveler could theoretically introduce the virus into local mosquito populations under favorable conditions.

Testing Capacity Is Limited

One challenge for physicians is that Zika testing is not as simple as ordering a routine laboratory panel. The CDC and specialized public-health laboratories perform molecular and antibody tests, and the appropriate test depends partly on how much time has passed since symptoms began.

Reverse-transcription polymerase chain reaction testing can identify viral RNA during the relatively brief period when the virus is present in blood. Antibody testing becomes more useful later, but interpretation can be difficult in people previously infected with or vaccinated against related flaviviruses.

Because of those limitations, the CDC advises clinicians to work through state and local health departments. Public-health agencies can help determine whether a patient meets testing criteria and can coordinate specimen submission.

The narrow testing window and uncertainty surrounding antibody results mean that physicians may sometimes have to make decisions with incomplete information. That is one reason the agency is emphasizing prevention and travel counseling.

Microcephaly Reports Drive Urgency

Brazil has become the center of international concern. The country has reported thousands of suspected microcephaly cases since the Zika outbreak expanded, although health authorities are still working to confirm how many meet diagnostic criteria and how many are associated with maternal infection.

Researchers are examining tissue samples, pregnancy histories and geographic patterns to determine whether Zika is a causal factor. Evidence reported so far is concerning enough that public-health agencies are treating the possible link seriously, but important questions remain unanswered.

Scientists do not yet know the risk to a fetus when a pregnant woman becomes infected, whether one trimester is more dangerous than another or why some exposed pregnancies may be affected while others are not. They also do not know whether infection before pregnancy creates ongoing risk.

Those uncertainties make counseling difficult. Physicians must explain a potentially serious risk without overstating scientific conclusions that have not yet been established.

U.S. Health Systems Prepare for More Questions

Obstetricians, primary-care physicians, emergency departments and infectious-disease specialists are likely to see increasing numbers of patients asking about Zika as winter travel continues to the Caribbean and Latin America. Healthcare systems will need clear pathways for identifying travel exposure, contacting public-health departments and arranging maternal-fetal follow-up when necessary.

The issue also places pressure on laboratories and public-health agencies. If travel-associated cases increase, demand for testing and counseling may expand quickly even in areas without local transmission.

The CDC says its guidance will evolve as more evidence becomes available. For now, the recommendations are built around three principles: reduce exposure during pregnancy, identify women who may have been exposed, and monitor pregnancies when infection is suspected or confirmed.

Zika is still a relatively unfamiliar virus to many American clinicians and patients. This week’s guidance makes clear that it is no longer merely a distant tropical-disease issue. The possibility of pregnancy complications has moved it into routine conversations about prenatal care, international travel and mosquito prevention, even as scientists continue working to answer the most important questions about the true level of risk.