The rate of young adults without health insurance has dropped because of President Barack Obama 's health care reform law, but expanded coverage for the age group could disappear if the Supreme Court strikes down the statute, a Gallup poll out Friday shows.

When health care reform became law in March 2010, 28 percent of Americans aged 18-25 did not have health insurance. During the first quarter of this year, that rate had fallen to 23 percent . Obama's law allows people under age 26 to remain on their parents' insurance plans, which has provided health benefits to a population that historically has been less likely to have coverage.

The Supreme Court is expected to issue a decision by the end of this month on whether Obama's health care reform law is constitutional . Justices could uphold the law, invalidate the individual mandate that most people must obtain health coverage or other provisions, or strike down the entire statute. Under the last scenario, young adults would lose the guarantee of coverage they now have through their parents' insurance.

"The young adults who appear to have taken advantage of the provision allowing them to stay on their parents' health insurance until age 26 may lose their coverage. Thus, these young adults will be even more reliant on an upturn in the economy for access to health insurance," according to Gallup.

Overall, 16.9 percent of Americans have no health insurance , Gallup reports. The Census Bureau estimates that almost 50 million people in the U.S. are uninsured . Young adults continue to have higher rates of uninsurance than older Americans. Because Medicare covers anyone aged 65 or older, just 3 percent of senior citizens had no health insurance during the first three months of this year. The uninsured rate for working-age adults between 25 and 64 years old was 19.6 percent in the first quarter, which is almost five percentage points higher than in January 2008, according to Gallup.

Health insurance coverage translated into better access to medical care in states that enacted their own laws for young adults prior to the national reforms, according to a recent study from the Mount Sinai School of Medicine in New York. It doesn't fix everything, however, as the Commonwealth Fund illustrates in a report issued Friday . Costs remain burdensome even for young adults with health insurance: 36 percent of people aged 19 to 29 reported they had difficulty paying medical bills or were in debt because of health care expenses.

There's a chance young adults might not lose the health coverage they get through their parents, even if Obama's law is repealed. Health insurance companies may be reluctant to dump young adults because their parents, who are the paying customers, like the policy. Likewise, some congressional Republicans have indicated that they would attempt to restore this popular provision of health care reform, though the party remains divided about how, or whether, to respond to a potential repeal of Obama's law.