Also, Gallup-Healthways points out that "[s]everal insurers have announced that they are abandoning some health exchanges in 2018, suggesting that coverage options could diminish and premiums could surge in the near future in some states." That may also have played a part in lower enrollment numbers.

Even though the uninsured rate has risen in the quarter just past, the poll finds that the number of U.S. adults without health insurance is still 6.7 percentage points lower than it was at its peak in the third quarter of 2013.

At that time it hit 18 percent—just prior to the opening of the health insurance exchanges in October of 2013. And beginning in 2014, the ACA's individual mandate required the purchase of coverage or payment of a tax penalty.

Young adults have been doing much better since 2013, with the percentage of uninsured among them falling sharply. The poll finds that adults aged 18–25 have seen their uninsured rate fall more than 7 percent—a circumstance possibly owing, at least in part, to a provision of the ACA that allows young adults to remain on their parents' health insurance until age 26.

Meanwhile, those aged 26–34 have seen a nearly 10-point drop in their uninsured rate.

That's important—even vital—to smooth functioning of the health insurance market, since their premiums help offset the higher costs of older Americans who typically use more medical services.

Uninsured rates are also down substantially among Hispanics and lower-income Americans; both groups had the highest uninsured rates before the mandate to carry insurance took effect. And even though the uninsured rate among Hispanics is still the highest of any major racial or ethnic group, at 28.6 percent, it's fallen 10.1 points since the last quarter of 2013.

Among non-Hispanic black and white adults, uninsured rates are down by nearly eight and five points, respectively. Among lower-income adults, it's dropped almost nine points; that's more than the decrease among middle-income (down 3.2 points) and high-income adults (down 2.4 points).

But the biggest changes come among self-paid plans and Medicaid, where adults who fully pay for their own plans have grown as a group by a little over three points since Q4 2016—from 17.6 percent to 20.7 percent. That probably indicates purchases of health coverage on the marketplace insurance exchanges introduced by the ACA.

Medicaid shows the second-largest increase among insurance sources, with an increase of nearly two points. Lower-income and disabled Americans have been able to buy coverage in the wake of the ACA's Medicaid expansion provision, which offered additional federal funding to states that expanded Medicaid eligibility.

Because of the possibility that the increase in the uninsured rate can be chalked up to the new administration's actions—and/or lack thereof—the study says that it "will be important to monitor the uninsured trends in the coming months as these events unfold. If coverage options and premiums do change, members of Congress from both parties may show a renewed urgency to address the health care issue."