AIDS would rank as the 15th leading cause of death in the United States if it were counted as a separate category, according to new figures from the National Center for Health Statistics.Acquired Immune Deficiency Syndrome killed between 12,450 and 13,820 Americans last year, the center estimated in its first annual summary including separate figures for the disease.

While AIDS has not yet been added to the rankings of major causes of death published by the agency, the 1987 toll would have placed it 15th on the list, ahead of birth defects, the agency noted.

The center's annual summary of vital statistics also disclosed that in 1987, America had the most births in nearly 25 years and the most deaths ever, and that marriage and divorce rates were at the lowest level in more than a decade.

The report said that 65 percent of AIDS victims were white males and 25 percent black males with between 4 percent and 5 percent each being black and white females.

The age groups most affected were 25 to 34 and 35 to 44, the study added.

AIDS is a contagious disease that attacks the body's immune system, rendering it incapable of resisting other diseases and infections.

The disease is spread most often through sexual contact, needles or syringes shared by drug abusers, infected blood or blood products, and from infected pregnant women to their offspring. The chief victims have been homosexual men and intravenous drug users.

As of Aug. 8, AIDS had been diagnosed in 70,208 Americans, of whom more than half, or 39,620, have died, according to the Centers for Disease Control.

A slight increase in fertility, coupled with the huge number of women in their prime childbearing years, resulted in the estimated 3,829,000 babies born last year, the most since 1964, the report showed.

Millions of people born in the post World War II Baby Boom are now in their 20s and 30s and, after completing school and launching careers, have begun families.

Deaths in 1987 totaled 2,127,000, up from 2,099,000 a year earlier and the most ever recorded for a single year in the United States. Record numbers of deaths have become common in recent years as the nation's population includes an ever larger share of elderly people.