July 25 (Reuters) - Centene on Friday posted a surprise quarterly loss, partly hurt by higher medical costs related to its insurance plans, sending its shares tumbling 9.1% in premarket trading.

The health insurance industry has seen elevated medical costs and changing enrollment patterns, at a time when it braces for big spending cuts and rule changes under the Trump administration.

"We are disappointed by our second-quarter results, but we have a clear understanding of the trends that have impacted our performance, and are working with urgency and focus to restore our earnings trajectory," CEO Sarah London said.

Rivals Elevance and Molina Healthcare have also warned of elevated costs in government-backed insurance plans.

Centene reported a medical cost ratio, the percentage of premiums spent on medical care, of 93% in the second quarter, compared with analysts' expectation of 89.34%.

The increase was driven by a reduction in net 2025 marketplace risk adjustment revenue as well as higher medical costs in Medicaid, driven mainly by behavioral health, home health and high-cost drugs.

Earlier this month, Centene withdrew its 2025 earnings forecast due to an expected revenue slump from commercial plans under Affordable Care Act, also known as Obamacare. These plans offer a sliding scale of government subsidies based on income.

For the second quarter, the health insurer reported adjusted loss per share of 16 cents, compared to analysts estimates of a profit of 86 cents, according to data compiled by LSEG.