In 1978, nearly 30 percent of all pregnancies in the United States uninterrupted by natural causes were terminated by legal abortions, according to a new study.

The study, which appeared in the latest issue of Family Planning Perspectives, estimated that there were 1,374,000 legal abortions in 1978, up about 54,000 over 1977.

The 1978 figure represented 28.9 percent of all pregnancies in the United States, excluding those ending in natural miscarriages. There were 3.3 million live births.

About one-third of the abortions were obtained by teen-agers. About three-quarters were for unmarried women.

Seven years ago, in 1973, the total of legal abortions was 745,000. But in that year, the Supreme Court struck down state antiabortion laws.

Since then, according to Dr. David Grimes of the U.S. Center for Disease Control in Altanta, the number of illegal abortions, which was estimated at several hundred thousands a year (the exact figures are unknown), has shown a "dramatic decrease," while legal abortions have shot up. Illegal abortions today are estimated at 10,000 or fewer a year, he said.

Figures for 1977, the latest available by local jurisdiction breakdowns, show that the District of Columbia had by far the highest incidence of abortion to total pregnancies (excluding those that ended in natural miscarriages). About 59 of every hundred such pregnancies resulted in legal abortions. New York was second at 44. The figure was 33 for Maryland and 29 for Virginia. Mississippi had the lowest incidence -- 5.5 per hundred.

The figures for 1977 show 31,620 legal abortions in the district, 23,700 in Maryland and 28,180 in Virginia.

The D.C. figures are so high, the report in Family Planning Perspectives said, in part because of the district's high concentration of "young black and unmarried women, groups which traditionally have the highest abortion rates," and in part because many women come from other jurisdictions to have abortion in the District because of the availability and ease of such services there.

In Maryland, on the other hand, almost as many women leave the state to have abortions elsewhere as remain there to have them. A few come in from outside.

Experts for the Alan Guttmacher Institute, which conducted the study, said that if the incidence of legal abortion is adjusted strictly to resident population and includes all residents who have abortions there or elsewhere than Maryland's incidence jumps to 46 per hundred pregnancies (the nation's highest), and the district drops to about 41, putting it in third place, just behind New York.

The study shows that more than nine-tenths of all abortions were in urban areas where abortion services tend to be more plentiful, and more than nine-tenths occur in the first three months of pregnancy.

The study estimated that if money and hospitals or clinics were available everywhere, another 479,000 pregnancies would have been ended by abortions in 1978.

Although Congress has cut off funds for most abortions under the federal-state Medicaid program for low-income women, the number of legal abortions has continued to rise, because at least 11 jurisdictions, including Maryland and the District are continuing to fund abortions for the poor on their own.

Another 12 states with more restrictive laws (including Virginia) have been ordered to continue funding abortion, at least for now, by the courts.