UnitedHealth is planning to pull out of its third Obamacare market, this time in Michigan.

The nation's largest insurer already had said it would leave Obamacare marketplaces in Arkansas and Georgia, and now Michigan officials told Bloomberg the insurer will exit that market as well. UnitedHealth has posted annual losses of more than $425 million due to Obamacare and has spoken of leaving all of the exchanges.

The moves come amid growing concern about the stability of the insurance exchanges as other insurers have had problems posting a profit.

However, insurance giants such as Anthem have remained committed to the exchanges and hope the markets will eventually become profitable.

UnitedHealth is expected to divulge more details on its Obamacare business when it reports first-quarter financial results on Tuesday.