UnitedHealth Group beat first-quarter expectations and hiked its 2019 forecast, but shares of the nation's largest health insurer continued their slump as investors worry about growing Congressional scrutiny of how the company and its competitors do business.

Shares of health insurers and pharmacy benefit managers, or PBMs, have bounced up and down this year, as Democratic presidential candidate Bernie Sanders launched a renewed push for a "Medicare for All" plan that could replace private coverage and as Congressional committees grilled PBM executives over their role in soaring drug costs.