WASHINGTON — Aetna will complete its withdrawal from Affordable Care Act insurance exchanges for 2018, announcing on Wednesday that lingering financial losses and uncertainty about the marketplaces’ future was prompting it to exit two final states.

According to an Aetna spokesman, the insurer will not sell individual health plans next year in Delaware or Nebraska. Its announcement came a week after the company said it would stop offering ACA health plans in Virginia in 2018 and a month after it said it would leave Iowa.

The cascade of state-by-state decisions represents a stark turnabout for the nation’s third-largest insurer, which initially entered 15 states’ marketplaces but last summer decided to slash its 2017 participation to just four. That retreat was the largest by any insurer from the health care law’s marketplaces.

But insurers have discovered that their ACA health plans tend to attract too few of the young and healthy customers needed to offset the expense of covering older people with medical problems. Aetna and other insurers have repeatedly reported financial losses on that part of their business.