Centene’s stock suffers a historic plunge as underestimation of healthcare costs leads the insurer to withdraw its full-year earnings outlook.

Centene shares tumbled 38% on Wednesday, on track for their largest single-day plunge, after the U.S. health insurer pulled its 2025 earnings forecast due to an expected slump in revenue from commercial plans under Obamacare.

Centene announced it was scrapping its full-year financial forecast after discovering fewer people were signing up for Obamacare health plans, and those that did were less healthy than expected. As a result, CNC was bracing for a $1.8 billion shortfall in risk adjustment revenue.