U.S. health officials are strongly urging doctors to ask all pregnant women about a possible Zika infection at every checkup.

So far, there have been no confirmed cases of a Zika infection from a mosquito bite in the United States, although officials expect mosquitoes will start spreading it in Southern states.

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All U.S. illnesses have been connected to travel to areas with Zika outbreaks.

The advice came Monday as the Centers for Disease Control and Prevention fine-tuned its guidance. It urged doctors to at least ask pregnant women if they or their sex partner were in an outbreak area, and suggested expanded use of a sophisticated blood test.

The Zika virus is mainly spread by mosquito bites, but also through sex. Infection during pregnancy can result in birth defects.