Almost 1 million employers, or 19 percent, plan to extend health care coverage to their employees' adult children before they're required to do so next year, according to new research from Hewitt Associates.

Of those early adopters, 10 percent will extend coverage to all eligible dependents; the remaining employers will do the same for graduating students. Hewitt estimates the average employer might add between five and 10 more adult children than they are already covering. The additional costs could be between $350,000 and $720,000 annually in premiums and claims costs.

The majority of employers are waiting to extend coverage until they are legally required to do so, though 4 percent have not yet decided when they will begin covering their employees' adult children. Among the reasons for waiting, Hewitt found are the cost and complexity of early adoption, but employers also feel the extended coverage just isn't necessary.