** Shares of health insurer Centene CNC.N fall 3.6% to $63.86

** Co to offer buyouts to most employees to cut costs after health plan membership fell sharply over the past year, Bloomberg News reports, citing a company spokesperson

** Layoffs could follow if voluntary departures do not meet CNC's target, report says

** CNC's Marketplace membership fell to 3.6 million in Q1 from 5.6 million a year earlier, according to CNC's Q1 results

** Centene did not immediately respond to Reuters request for comment

** Including session's move, CNC up 52.9% YTD