Nearly a third of the nation's 15-to-19-year-old women have has sexual intercourse, an increase of almost a third of female adolescent sexual participation since 1971, two John Hopkins researchers reported yesterday.

These unmarried young women are having sex "in greater numbers, at earlier ages and with more partners" that at the start of the decade, a nationwide survey showed.

Also, these young women now mainly have sex in the home rather than in the car or in the outdoors - at the male's home most often, at a relative's or friend's next often and in their own home next.

At age 15, they found, less than a fifth of young women had ever had intercourse. But by age 17, the number was more than 40 per cent and by 19 more than 55 per cent.

There has been much previous speculation about the number of young women who have experienced sex, and some limited studies. But the Johns Hopkins studies of practices in ones that have attempted to learn the facts nationwide for a broad cross-section representing all social and racial groups.

In the latest study the Johns Hopkins professors questioned 1.901 "never-married" 15- to 19-year-old women all over the country to get what they call "a national probability sample," the kind and size of sample that has proved generally accurate in many fields. It was drawn to try to represent the nation's 15- to 19-year-old female population accurately in geography, race and age; they all made the 1971 study.

The researchers - Melvin Zelnik and John F. Kantner - stated no moral or ethical judgements on the numbers their survey produced, or what Zelnik called the "surprising" fact of much teen-age sex in the home.

But Kantner told a news conference, "I think that if parents look at these results, they will be very surprise at what's going on under their noses."

Dr. Louise Tyrer, medical vice president of Planned Parenthood-World Population, in whose monthly journal the Zelnik-Kantner report appears, was more explicit.

"It behooves parents to take off their blinders and realize children are becoming more and more sexually active at younger ages," she said.

She said both parents and youths need to take "a more realistic approach" and focus on avoiding the unwanted pregnancy" the real disaster."

The professors did find a marked increase in the sexually active girls and young women who use contraceptives - 3 in 10 said "always," compared with fewer than 2 in 10 five years earlier; 64 per cent reported using a contraceptive at the time of last intercourse, compared with 45 per cent five years earlier.

The number using the pill and IUD (intra-uterine device), rather than condoms or ineffective "withdrawal," was also up.

At the same time, however, the number of the sexually active females who said they "never" used contraceptives was up too - from 17 per cent in 1971 to 26 per cent in 1976.

"I was surprised by the small in-roads sex education has had." Zelnik commented. "More than half the girls are ill-informed in terms of knowing when during the menstrual cycle pregnancy is likely to occur.

"Those that have had a sex education course are better, but even among the more than half are misinformed. I suspect that whatever is coveredin sexeducation courses, they're not very good."

Zelnik and Kantner are professors of population dynamics in the Johns Hopkins School of Hygiene and Public Health, and their study was financed mainly by the National Institute of Child Health and Human Development, part of the federal National Institutes of Health.

Asked by one reporter why they were studying teen-age sex, Zelnick pointed out the obvious: Sex is the first step toward pregnancy, and pregnancy is the first step in population dynamics.

The researchers found that the age of first sex for women has moved slightly downward, from an average age of 16.5 in 1971 to a recent 16.2.

Fifteen per cent of the young women, mainly those aged 15 to 17, had had sex only once. Forty per cent of the sexually experienced (compared with a larger 48 per cent in 1971) had not had sex at all in the four weeks preceding the interview. Just 27 per cent had had intercourse as many as three times in the previous four weeks.

In what they called the "geography of premarital sex" - an area that has previously received little academic attention - the professors found that three out of four of both blacks and whites had sex in the home.

But if in was not to use a hotel or motel, whites and automobile or "elsewhere," which the researchers translated as "some part of the 'great outdoors.'" The called this "an option somewhat less open to blacks, who are more urban."