Actually, although Norplant itself was approved for use in the United States only in late 1990, the technology behind it is not really new at all. It's only the delivery system that's new. The active ingredient is a synthetic female hormone called levonorgestrel-one ofthe active ingredients in many oral contraceptives in common use today. Instead of having to take a pill every day, though, a woman has six small capsules about the size of paper matches implanted just beneath the skin on the inner side of her upper arm - and that's that. The capsules, made of a type of silicone rubber called Silastic, diffuse the hormone into the bloodstream very slowly, maintaining levels that are high enough to prevent pregnancy for five years. (One other advantage: With Norplant, blood levels of hormone are kept at considerably lower levels than with most oral contraceptives.)

Implanting the capsules is quite simple. (Doctors prefer to do it toward the end of a woman's period, to make sure she isn't already pregnant.) A small incision (about I/S inch long) is made in the skin, and then the capsules are inserted in a fan-shaped array, so that they can all be removed through the same opening. You can't actually see the capsules once they're implanted, but you can feel them and sometimes see their outline on the surface of the skin. They shouldn't move around much at all, no matter how active you are. Taking them out is a little more involved and may take 30 minutes or more. If you want to keep using Norplant after your first five-year stint is up, a fresh set of implants can be inserted through the same incision.

Norplant seems to prevent pregnancy in much the same way oral contraceptives do, mainly by suppressing ovulation. It also thickens the mucous plug that keeps a lid on the cervix, making it more difficult for sperm to penetrate the uterus and tango with an egg. Norplant also suppresses development of the uterine lining - the endometrium - making it more difficult for the egg to become implanted in the uterine wall.