LOS ANGELES — Nearly 18 months after the passage of the national health care overhaul, American employers say they are providing health benefits for a growing number of people as they extend coverage to their workers’ adult children, a new survey finds.

The federal health care law allows young adults up to age 26 to stay on their parents’ health plans. As a result, employers say they have seen an average 2 percent increase in insurance enrollments, according to the survey by benefits consulting firm Mercer.

Many employers expect the ranks of their covered employees to grow an additional 2 percent in 2014 as other provisions of the law take effect that require companies to automatically enroll full-time workers.

The survey of nearly 900 U.S. employers did not put a dollar figure on the cost, but Mercer said the expanding levels of coverage are saddling employers with new financial pressures.