RIO GRANDE CITY — First it was a trickle, now it's a flood. Rising numbers of undocumented immigrants from Mexico and Central America are streaming into Texas to give birth, straining hospitals and costing taxpayers hundreds of millions of dollars, health officials say. Doctors and health officials say they are overwhelmed by both the new arrivals and those immigrant mothers who already are in the state. Even Houston's feeling the pinch. An estimated 70 percent to 80 percent of the 10,587 births at Ben Taub General Hospital and Lyndon B. Johnson General Hospital last year were to undocumented immigrants, administrators...