Two-thirds of young American women have sexual intercourse by age 19 and nearly all the sexual activity is premarital, according to a new study by three population experts at the Johns Hopkins School of Hygiene and Public Health.

The study, just published in the new issue of Family Planning Perspectives, is based on a representative sampling of 2,193 women in the year 1976.

It shows a sharp increase in the incidence of sexual intercourse amoung teen-age women since 1971, when the researchers conducted their pioneering study of teen-age sexual activity.

The 1971 survey showed that about 55 percent of young women had sexual intercourse by age 19. The 1976 figure was 63.3 percent, about two-thirds. The biggest increase was among white teen-agers, where the figure jumped nearly 10 percentage points over the five-years period and reached 60 percent in 1976. The figure that year for black teen-agers was 82.8 percent.

The researchers, professors Melvin Zelnick and John F. Kantner and assistant professor Young J. Kim, also said that one in 10 U.S. women becomes pregnant before age 17, one in four before she is 19, and that eight in 10 of these pregnancies are premarital.

Richard Lincoln, editorial director of the magazine, said the increase in sexual activity by teen-agers -- the figures show that one in five women has intercourse as early as age 16 -- reflects "the greater sexual permissiveness that has been going on for a long time." The increase in sexual activity began even before the so-called sexual revolution of the 1960s and has been continuing, he said.

The 1971 and 1976 survery were funded by grants from the Department of Health, Education and Welfare.

The Kantner/Zelnick/Kim studies in 1976 calculated that the median age of young women at the first act of sexual intercourse was 16.6 years for blacks, 18.4 for whites and 18.1 for white and black combined. In 1971, the combined figure was 18.7 years, which means that teen-aged girls are having first intercourse at an earlier age.

Although a quarter of these young women get pergnant at least once by age 19, not all these pregnancies result in live births because of miscarriages and abortion. Three-quarters of those who get pregnant actually have the babies, the study showed.

Another article in the magazine, written by Christopher Tietze of the Population Council, reported that legal abortions for women of all ages jumped from 193,500 in 1970, when New York and three other states authorized legal terminations of pregnancy, to an estimated 1,270,00 in 1977.

Government officials say that the greater incidence of teen-age sexual intercourse has brought with it a flood of teen-age pregnancies, estimated at 1 million a year. Many of these young women have miscarriages or abortions, but about 600,000 annually give birth and most keep the babies. Nearly half the births are to unmarried women.

John Robbins of SRI International calculated last May 11 that teen-age pregnancies cost the American taxpayer about $8.3 billion a year in welfare and related outlays.

Family Planning Perspectives is published by the Alan Guttmacher Institute, a supporter of family planning and contraception.