The national abortion rate - the number of abortions per 1,000 women - dropped 6 percent between 1980 and 1987 as the overall number of abortions has stabilized at about 1.6 million per year, a study by the Alan Guttmacher Institute said.

The study released this week said the overall abortion rate dropped from 29.3 abortions per 1,000 women to 26.9 per 1,000 in the 1980-1987 period.Another measure, the abortion ratio - the percentage of pregnancies that end in abortion - also showed a decline in the 1980-1987 period, dropping from 30 percent to 28.9 percent.

But the study, reported in the institute's journal, Family Planning Perspectives, also found that over the same period, the abortion rate among women under 15 years old increased 18 percent - from 8.4 to 9.9 per 1,000 women - and 11 percent among minority women aged 15 to 19, from 66 to 73 abortions per 1,000 women.

The study also found that the gap in the ability to to prevent pregnancy is widening between minority, mostly black, and white teenagers.

It found that the overall teenage pregnancy rate stayed about the same during the 1980s but declined among white women aged 15-19, dropping from 96 to 90 pregnancies per 1,000 women despite an increase in the proportion who said they were sexually active.

Among minority teenagers, however, the pregnancy rate increased 2 percent between 1980 and 1987, despite dropping to a low of 181 per 1,000 in 1984, reaching a rate of 189 pregnancies per 1,000 in 1987.

The authors of the study - Stanley Henshaw of the Guttmacher Institute, and Lis Koonin and Jack Smith of the Centers for Disease Control - said the overall decline in the national abortion rate was not caused by a reduction in the rate of unintended pregnancy.

"We cannot determine the exact reasons for the abortion decline," they said. "It could be because of reduced access to abortion services or changed attitudes toward both abortion and carrying unplanned pregnancies to term.

"Some unmarried women facing an unintended pregnancy may not feel as desperate as they would have a few years ago when non-marital childbearing was less accepted," they added.

Higher abortion rates and birthrates among women younger than 15 is probably a reflection - at least in part - of the growing rate of sexual activity among teenagers, the study said. The proportion of sexually active 15-year-olds rose from 21 percent in 1982 to 28 percent in 1988.

As in the past, the groups of women having the highest numbers of abortion in 1987 were predominantly young, unmarried and white. Of those having abortions, 80 percent were under age 30; 82 percent were not currently married and 65 percent were white.

But, among sub-groups, women with the highest rates - rather than numbers - are whose who are young, unmarried and non-white.

The study said the 6 percent overall decline reflected a major drop in the rate among married women older than 25 and among white unmarried women of all ages.

It found the abortion rate among all white women fell from 24 to 21 per 1,000 women in the 1980-1987 period, while the rate among all non-white women stayed around 56 per 1,000 in the same period.