The U.S. Centers for Disease Control and Prevention on Friday revised its guidelines for pregnant women to include a recommendation that even those without symptoms of the Zika infection be tested after returning from affected areas.

The updated guidelines recommend pregnant women be offered testing 2 to 12 weeks after returning from affected areas, while the agency had earlier suggested tests for those already experiencing symptoms.

Experts have said that since 80 percent of those infected by the virus show no symptoms it leaves many women no way of knowing early enough to make an informed choice about their unborn child.

Although sexual transmission of Zika virus is possible, mosquito bites remain the primary way it is transmitted, the agency said on Friday.

There is no vaccine or treatment for Zika, which has caused outbreaks in at least 26 countries in the Americas.

Brazil is investigating more than 4,000 suspected cases of microcephaly, a condition marked by abnormally small head size that can cause permanent brain damage in newborns, that may be linked to Zika.

The suspected link appears "stronger and stronger" as researchers study whether there is a causal connection, the head of the CDC said on Friday.

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The agency also issued new guidelines for both pregnant women and women of reproductive age who reside in affected areas.

Pregnant women experiencing symptoms should be immediately tested, and those not, are recommended to when they begin prenatal care, the CDC said.

Dallas County reported on Tuesday that the first known case of Zika contracted in the United States was a person infected after having sex with someone who had returned from Venezuela.