The problem of teen-age pregnancy remains a staggering one. The statistics are painfully familiar: more than 1 million pregnant U.S. teen-agers in 1980, giving the country one of the highest teen pregnancy rates in the world. Washington, D.C., where nearly 10,000 teen-agers become pregnant each year, has one of the highest rates in the nation.

If current trends in teen-age sexual activity and contraceptive use continue, 39 percent of today's 14-year-olds will have at least one pregnancy before they turn 20. Perhaps the most startling statistic: 36 percent of the first, premarital pregnancies occur within three months of the time a girl first has intercourse.

Behind these statistics lie some recent changes in teen-age contraceptive practices. Melvin Zelnik and John Kantner of Johns Hopkins, leading researchers in the field, point out that "teen-agers are trying harder than ever to avoid pregnancy and childbirth outside of marriage."

In 1979, 49 percent of sexually active teen-age girls said contraception was used the first time they had sex, compared with 38 percent in 1976. But in two out of five cases their partners used a condom or withdrawal, the least effective methods. Use of the pill or an IUD as the first method of contraception dropped 41 percent between 1976 and 1979, while reliance on withdrawal and rhythm increased 86 percent.

So teen-agers may be trying harder, but they are turning away from the most effective methods. Concerns during the late 1970s about the pill's safety could be behind a good deal of this change in teens' contraceptive practices.

Meanwhile, the experts on teen-age birth control are looking to the new, nonprescription contraceptive sponge as a possible antidote to the high teen-age pregnancy rate.

"The sponge will have particular appeal for young people," claims Dr. Louise Tyrer, Planned Parenthood's vice president for medical affairs.

One survey of clinic patients found that nearly half (44 percent) of sexually active teens report periods of abstinence of four months or more. For girls who are not in a long-term relationship, say the experts, an over-the-counter nonprescription method may be particularly appealing.

"Nonprescription over-the-counter methods are generally very appropriate for adolescents," says Judy Senderowitz, executive director of the Center for Population Options, a Washington group set up in 1979 to meet teens' needs for birth-control information.

On average, teen-agers wait nine months after their first sexual experience before visiting a family planning clinic to get help. And when girls do get to a clinic, 40 percent are there because they think they're pregnant, and nearly a third say they didn't come earlier because they were afraid their family would find out.

The sponge seems to have an advantage over foam or a diaphragm in that it provides continuous protection throughout 24 hours, an important consideration for teen-agers, says Senderowitz.

One drawback of the sponge is that it's not as effective as the pill or an IUD. But its manufacturer, VLI Corporation of California, claims its "user-effective" (as opposed to theoretical) rate is the same as the diaphragm when used with a spermicide: 85 percent. And once a teen-age girl uses any birth-control method, studies have shown, she is likelier to get a more reliable method than someone who is unprotected.

Another possible disadvantage is cost. With a suggested retail price of $1 each, the sponge may be too expensive, says Senderowitz, for some sexually active teen-agers.

Now on sale in 12 western states, the sponge (marketed under the name Today) should be in Washington-area drugstores early next month. VLI Corp. has a national advertising campaign planned for the fall, including a toll-free number to be called for a free trial pack or for advice. According to VLI president Dr. Bruce Vorhauer, advertising will be aimed at 25- to 34-year-old women.

The safety of the new birth-control method and the procedures FDA followed when approving it were questioned at recent hearings of the House subcommittee on intergovernmental relations and human resources. Outgoing FDA Commissioner Arthur Hayes testified that the "risks associated with the use of Today Contraceptive Sponge were carefully and thoughtfully considered in approving this product for marketing. We believe it is a product which provides another safe and effective alternative method for women desiring contraception."