Shares of Centene (NYSE: CNC) have tumbled more than 20% after the healthcare giant stunned investors by withdrawing its full-year financial guidance. The move echoed a similar shock from UnitedHealth Group (NYSE: UNH) in May, deepening concerns about rising costs and mounting pressure across the health insurance sector. In pre-market trading on Tuesday, Centene stock dropped as much as 26% to $41.57. This followed a previous close of$56.65, where CNC shares had gained 4.3% in the prior session. CNC one-da ...