NEW YORK (AP) – 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.

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.