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The Centers for Disease Control and Prevention is urging pregnant women who may have been exposed to the Zika virus to be tested for up to 14 days after their symptoms begin — a longer period than the agency had previously recommended.

The new guidance, issued Monday, also applies to pregnant women who have no symptoms. The agency is updating its guidance because of new research showing the virus can stay in the blood of pregnant women for longer than the previously recommended seven-day window for testing after symptoms begin. Even pregnant women without symptoms can have evidence of the virus in their blood and urine, the CDC said.

The diagnosis of Zika infection is complicated. Many people infected with Zika won't have symptoms or will have only mild symptoms. Until now, scientists assumed that if a person becomes sick after being infected, the virus stays in the person's blood for about a week after symptoms started. That's why the first week of illness was considered the best time to find evidence of the virus in the blood using a Zika-specific test.

There is another test that looks for evidence that the body is fighting a virus in the family of viruses that includes Zika. The results of that test, typically performed more than a week after symptoms started, are harder to interpret and may not provide a definite diagnosis. That's because the body's reaction to Zika can resemble the reaction to similar viruses, especially in countries where other diseases like dengue and chikungunya, which are viruses in the same family as Zika, are common.

The CDC's guidance on Monday recommends a longer period to test for Zika virus particles in the blood of pregnant women, which officials hope will provide a definite diagnosis for more pregnant women infected with Zika. If pregnant women visit their health-care provider after the 14-day testing window and test positive with the less-specific test, they can be offered the Zika-specific blood testing to provide a definite diagnosis.

The virus is primarily transmitted through the bite of an infected mosquito, but it can also be spread through sexual contact. Until recently, scientists thought men spread the virus to their sexual partners. But after a recent case of female-to-male sexual transmission, the CDC is also updating its recommendations for the prevention of sexually transmitted Zika to include the possibility of sexual transmission from an infected woman.

In updated guidance released Monday, the agency recommends that all pregnant women with sex partners, male or female, who live in or traveled to an area with Zika use condoms during sex or abstain from sex for the remainder of their pregnancy.

There are at least 400 pregnant women infected with Zika in the 50 states and the District of Columbia, and another 378 pregnant women in the U.S. territories, most of them in Puerto Rico.

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