UnitedHealth is the first insurer to report results every quarter, and many on Wall Street see it as a bellwether for the sector. Shares of several other insurers plunged in early trading.

UnitedHealth Group surprised investors with what its CEO said was an "unusual and unacceptable" quarterly earnings miss, and it lowered its outlook for the full year due to higher-than-expected medical costs,

UnitedHealth said it was blindsided by rising medical costs as the first quarter closed, upending a forecast it had affirmed just three months ago.