One of every four pregnancies in the United States in 1981 ended in abortion, according to a report yesterday from the Alan Guttmacher Institute, the nonprofit research organization associated with Planned Parenthood.

The report said U.S. women obtained abortions in 1,577,340 out of 6.1 million pregnancies in 1981 -- a rate of 26 percent, about the same as the year before. The report also said 85 percent of those abortions were by the suction method (vacuum aspiration).

About 51 percent were performed within eight weeks of conception, and 91 percent within three months of conception, it said.

Although white women, as the nation's largest racial group, obtained 70 percent of the abortions, the rate was much higher among nonwhites -- about 56 per 1,000 women for nonwhites compared with 24 for whites. The report said 81.1 percent of the abortions were obtained by unmarried women.

By age category, 1 percent of the abortions were obtained by girls under 15, a total of 15,000 abortions in this age group.

The report said young women 15 to 19 obtained 433,000 abortions, or 27.5 percent of all abortions.

Women age 20 to 24 obtained 555,000 abortions, or 35.2 percent of the total.

Twenty percent (316,000) of the abortions were obtained by women 25 to 29, 10.6 percent by those 30 to 24, and the remaining 5.7 percent by women 35 and over.

In general, the report said, the age distribution of abortions in 1981 differed little from 1980.

It said about 35 percent of the abortions were obtained by women who had had them previously; and it said 6 percent of the abortions were obtained by women outside their home states, in part because abortion services were not available in their home area.