Centene reported a $253 million loss in its second quarter as the provider of government-subsidized health benefits struggles to manage costs of its health plan members.

Centene on Friday posted a surprise quarterly loss, partly hurt by higher medical costs related to its insurance plans, sending its shares tumbling nearly 13% in premarket trading. The health insurance industry has seen elevated medical costs and changing enrollment patterns,

In its second quarter, the Clayton-based company lost $253 million dollars, 51 cents a share. In the same period last year, Centene saw earnings of $2.26 a share.