MIAMI (AP) — Yessica Flores is getting ultrasounds more frequently than the average pregnant woman, but there's a cruel downside accompanying the joy of seeing how much her unborn daughter grows every few weeks: fear of possible defects caused by Zika. [...] doctors don't currently know why the virus — which is mainly spread by city-dwelling mosquitoes but also can be sexually transmitted — remains detectable in a pregnant woman's bloodstream far longer than in the bloodstream of a man or a woman who is not pregnant. [...] this summer, the only cases of Zika on the U.S.