Unmarried white girls who have sex before age 15 are now twice as likely to become pregnant as in the mid-1970s, two Johns Hopkins University population experts reported yesterday.

The major reasons, they said in a new study, are "a sharp increase in the frequency of sexual activity" among these girls and a shift to less effective methods of contraception.

The researchers said that, according to a 1979 survey of more than 1,600 teen-age girls in urban areas, 39 percent of all unmarried white girls who had sexual relations before age 15 became pregnant out of wedlock within two years, compared with 20 percent in a 1976 survey.

The proportion also rose for unmarried black girls in the same age category, but not as dramatically. In 1979, just under 44 percent of them became pregnant within two years of first sexual experience, compared with 37 percent three years earlier.

The survey showed that most of the pregnancies among these girls, both white and black, occurred within the first nine months to a year after the start of sexual activity, when the girls were less likely to be familiar with contraceptive methods.

The report was written by Michel Koenig, postdoctoral fellow, and Prof. Melvin Zelnik, Department of Population Dynamics, of the Johns Hopkins School of Hygiene and Public Health.

The study is the latest in a series from Johns Hopkins on the remarkable change in the sexual habits of teen-age girls over the past generation. A Hopkins study published in 1980 detailed the sharp rise in sexual activity among teen-agers: In 1971, only half of all unmarried young women had sex by age 19; by 1976 the figure was 59.5 percent and by 1979 it was 69 percent.

Koenig, in an interview, said the new survey covered only metropolitan areas (which have three-quarters of the population); the results for rural areas would be somewhat lower, based on results of previous studies.

The survey showed that girls who begin sexual activity later in their teens are somewhat less likely to become pregnant, largely because they are more likely to use the most effective methods of contraception.