A second big shoe just dropped on Iowa's Obamacare marketplace.

Three days after Wellmark Blue Cross & Blue Shield said it would not sell new individual health insurance plans in Iowa in 2018, Aetna on Thursday said it also will not offer new Obamacare plans next year, citing "financial risk."

The moves leave Iowa with just two other insurers that, as of now, sell Obamacare plans in the state.

A third Iowa insurer, Medica, told CNBC is has not yet decided whether to continue selling plans there next year. Medica is the only company to offer individual plans in all of the state's counties. The fourth insurer, Gundersen Health Plan, said that its current plan is to participate in the Obamacare marketplace in Iowa.

Aetna said it is "still evaluating" if it will sell Obamacare plans in the three other remaining states where it currently does so.

Aetna covers about 30,000 people in Iowa who buy plans through the federal Obamacare exchange, HealthCare.gov, according industry sources.

Slightly fewer than 52,000 Iowans signed up for coverage on HealthCare.gov during open enrollment for 2017.

Wellmark covers about 21,000 people through Obamacare plans. But a number of them bought those plans outside of the federal exchange.

Aetna spokesman T.J. Crawford said, "Earlier today we informed the appropriate federal and state regulators that Aetna will not participate in the Iowa individual public exchange for 2018 as a result of financial risk and an uncertain outlook for the marketplace."

"We are still evaluating Aetna's 2018 individual product presence in our remaining states," Crawford said.

The remaining states are Virginia, Nebraska and Delaware.

The situation in Iowa is mirrored in a number of other states where insurers are seriously considering whether to exit Obamacare marketplaces.

Some insurers are worried about remaining in markets where they have been unable to make money as a result of too many customers with high health needs, and not enough health customers to offset their costs. Wellmark blamed its departure from Iowa's Obamacare market on $90 million in loss over three years.

Insurers also are worried about the future of Obamacare, particularly because Republican leaders in Congress have been unable, as of yet, to pass replacement legislation for the health-care law.

Wellmark CEO John Forsyth earlier this week said, "Finding solutions to stabilize this market is in the best interest of all Iowans, including providers of health care and insurance carriers."

"No one really benefits from rising costs," he continued. "While there are many potential solutions, the timing and relative impact of those solutions is currently unclear. This makes it difficult to establish plans for 2018."

Wellmark and Aetna's departures were announced about a month before insurers in Iowa have to start submitting their proposed premium rates for 2018.

About one-third of the counties in the United States this year are served by just a single Obamacare insurer.

Health-care advocates have worried about markets that currently have just one insurer potentially being left with no insurer in 2018, due the uncertainty in Washington. Humana's announcement that it was leaving the Obamacare exchanges in 2018 now leaves Knoxville, Tennessee with the prospect of no Obamacare insurers next year.

But Iowa, which had four insurers on HealthCare.gov in 2017, was not one of the states that analysts had been eyeing for a major market disruption.

"Iowa would not have been on our radar of states where they were at risk, necessarily. But then you have to two companies leave within a space of a couple of days, and suddenly it is at risk," said Cynthia Cox, associate director at the Kaiser Family Foundation.

"I think this underscores how much the political uncertainty can cause markets that were otherwise appearing to be stable, to become at risk almost overnight," Cox said.

The U.S. House Energy and Commerce Committee, which is controlled by Republicans, put out a press release on the heels of Aetna's announcement that noted Wellmark's earlier departure as well.

"The insurer exodus continues [its] drip-drip-drip effect on patients, leaving them guessing on the future of the marketplace," the release said. "This year, Iowans had four insurers offering plans on the individual market. Next year they will have just one state-wide insurer. Patients deserve better – a health care system that puts them first and delivers on being more affordable and accessible."

Larry Bussey, a spokesman for Medica, when asked what that insurer planned to do in light of Aetna's decision, said, "At this point we're just looking at the situation and evaluating what our options are, and really looking at what we're going to do."

Medica covers about 14,000 people in Iowa who buy plans though HealthCare.gov.

CNBC has reached out to comment from the fourth Obamacare insurer in Iowa, Gunderson.

In a note Thursday, Evercore ISI said that "the decline in Iowa's exchange sign-ups this year was a bit larger (-6.4 percent) than the U.S. overall (-3.7 percent), and it also saw a bigger move towards higher-cost older members."

"But clearly the exit of 2 of 4 carriers from Iowa now raises the uncertainty for exchanges in general in 2018," Evercore ISI said.