In a few weeks, American women will gain access to the first truly new birth control device to be offered in 25 years. The implantable contraceptive, called Norplant, does its job by tricking Mother Nature with a synthetic hormone that suppresses ovulation. Such meddling produces some bothersome side effects, but its benefits include an effectiveness that rivals abstinence.

Often described as revolutionary, Norplant is really not such a radical product. The synthetic female hormone it contains has been used in birth control pills for more than 20 years. The hormone's dispenser is a tube of rubbery plastic that has been used for more than 30 years in heart valves and to drain fluids from the brain.

Six of the tubes, each the length of a matchstick, are inserted under the skin, where they slowly release hormone for five years, or until the implants are removed.

The introduction of Norplant has already inspired a touchy debate over the role of implanted contraception and the possibility of its enforced use, especially among the poor or drug-addicted women. The first instance of a court-ordered implant of Norplant occurred last week in California.

Largely overlooked in the discussions, however, is how the device works and how women have reacted to it. Can others see the implants? Do they hurt? Alter sexual desire? Change bleeding patterns?

"I think Norplant has a place. But I don't think it is the optimal contraception for everyone," said Daniel Mishell, chairman of the department of obstetrics and gynecology at the University of Southern California.

Mishell ran one of three clinical trials to test the safety and efficacy of Norplant in the United States.

"It offers women another option, but it is not a panacea," said Mishell.

"For some women, it's great. It's convenient. You don't have to remember to take a pill. It's also very effective. But there are some problems too," said Maria Lacarra, the nurse at USC who assisted with Mishell's clinical trials.

Norplant, in one form or another, has been in development since 1966. Invented at the nonprofit Population Council's laboratories at Rockefeller University by Sheldon Segal and an international team of investigators, it is registered for use in 17 countries. The Food and Drug Administration approved the device in December, and starting next month, Wyeth-Ayerst Laboratories of Philadelphia will begin distributing the Finnish-made implants to doctors trained to insert them. The estimated cost of the procedure and the Norplant device, which lasts for five years, is between $350 and $600. Taking birth control pills for a year costs $235. Synthetic Hormone Crystals

The Norplant implants are 1.4 inches long and made of a medical-grade plastic called Silastic. The Silastic tubes are rubbery, soft and filled with powdered crystals of a synthetic progestin hormone called levonorgestrel. The ends are stoppered with more Silastic.

Six tubes are inserted just below the skin on the soft flesh of a woman's upper arm. Researchers have experimented with other locations, but they found the tubes were more visible on the lower forearm and tended to migrate when they were inserted into the buttocks. Segal said six tubes are needed to provide enough hormone to do the job.

"What does it feel like?" said nurse Lacarra, who had empty Norplant tubes put into her own arm. "It feels like six implants."

A study by Philip Darney at San Francisco General Hospital of 200 women with the implants found that three-fourths of the subjects reported little or no pain during the insertion or removal of the tubes. One in five women said that someone had noticed the implants.

As knowledge of Norplant grows, women could become more sensitive about advertising their reproductive status. All the implants can be felt under the skin, but they are soft and flexible. Darney reports that most women in his survey did not think of implants as a particularly weird form of birth control.

Immediately after their insertion, the tubes begin releasing hormone. The levonorgestrel permeates the plastic Silastic and makes its way to its outer surface, where it is carried away by the bloodstream. In the first weeks, the tubes release about 85 micrograms of hormone a day. By the end of five years, however, the flow is only about 30 micrograms. The most common form of birth control pill provides about 150 micrograms of synthetic progestin a day. Signaling the Pituitary

The hormone performs several tricks. "The mechanism of action is clear, but not simple," Segal said. First, the elevated level of synthetic progestin suppresses ovulation. It does this by signaling the pituitary gland in the brain to stop producing the two hormones that inspire follicles in the ovary to produce mature eggs. Yet Segal and his colleagues found that even with Norplant, in the first year 11 percent of all women's cycles still produce an egg. By the fifth year, as many as half the cycles may be ovulatory.

Norplant, however, provides a backup. The elevated progestin levels keep the mucus on a woman's cervix sticky and thick, which foils a sperm's attempt to reach the egg. Normally, the cervical mucus gets watery during ovulation, which helps sperm on their journey to what Segal calls the "arena of fertilization."

The combination of suppressed ovulation and sticky mucus results in Norplant's extremely low failure rate in preventing conception. Harold Nash of the Population Council's Center for Biomedical Research reports that if 100 women of all weights used Norplant for five years, four of them would become pregnant. Norplant is most effective for women weighing less than 153 pounds. The heavier the women, the greater the chance of pregnancy, though the likelihood of becoming pregnant with Norplant, even for heavy women at the end of five years, is still extremely low.

The device does have pronounced side effects, with many women reporting headaches, depression and irregular bleeding, which includes spotting, heavy bleeding and no bleeding. "The biggest problem with Norplant is irregular bleeding," said Lacarra. "A lot of women freak out with irregular bleeding. So this would not be for them."

The irregular bleeding caused some problems in the sex lives of the women studied by Darney. But of the women who said that their sexual activity had changed as a result of Norplant, two-thirds reported that it was a change for the better, because of less worry about pregnancy and more spontaneity in their lovemaking.

The first major new contraceptive since the 1960s offers an unusually high degree of protection against pregnancy for up to five years, and its side effects are easily reversed. But it also raises new social and medical questions for women considering it.

1. Six capsules filled with crystallized synthetic female hormone are inserted just beneath skin on inside of upper arm. The hormone - progestin - seeps at a controlled rate through capsule walls.

2. Bloodstream carries progestin to pituitary gland in brain in decreasing amounts for up to five years.

3. Within a day or two after implants are inserted, pituitary gland stops producing hormones required to signal ovaries to produce mature eggs. Progestin also keeps cervical mucus thick, locking sperm outside cervical canal.

In a usually painless 15-minute procedure, doctor begins with 1/8-inch cut. Although uppper arm is anasthetized, incision is too small to require stitch.

Hollow needle, or "feed tube," is inserted temporarily into incision to depth barely beneath skin's surface.

Using plunger, doctor pushes each of six rubbery capsules through the tube, swiveling it to a new position each time to create a fan-shaped array.

SOURCE: The Population Council