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.

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.