Obamacare is facing a new challenge after a wave of insurance companies have said they’re leaving the health law’s marketplaces — dwindling patient choices from the insurers who remain.

Blue Cross-Blue Shield nonprofit insurers are largely staying in Obamacare markets, but to contain costs, some are moving toward HMO plans with narrower networks. The shift is part of a growing trend on the exchanges as plans with broader networks are losing money.