When Norplant, a surgical birth-control implant, was introduced, its relative high cost -- $500 -- was a drawback. But now Maryland's largest health-care providers are covering this birth-control option. The state will pick up the $500 tab for Medicaid patients. Blue Cross and Blue Shield and two health maintenance organizations are covering the device, too. "Everything points to this being a real advance in controlling unwanted pregnancies," said Dr. Arthur Keefe of Blue Cross. Uninsured women may also get help: Planned Parenthood is exploring ways to defray the cost for them. Norplant is neither novel nor revolutionary. It is, though, an effective form of birth control that could give women new choices while reducing the number of reproduction-related surgical procedures.