The death rate among women 35 and older from pregnancy-related causes has dropped nearly 50 percent since the mid-1970s, a study shows.

"Although older women will probably continue to be at higher risk of maternal death, recent trends . . . should be reassuring to younger women who are considering postponement of pregnancy and to women aged 35 or older who are contemplating pregnancy," researchers said in the Journal of the American Medical Association.

The study said pregnancy-related deaths among women 35 and over were about 50 percent lower in 1982 than from 1974 through 1978.

The mortality rate among such women in 1982 was 24.2 deaths per 100,000 live births, compared with an average 47.5 deaths per 100,000 live births for 1974 through 1978, the study said. That is still three times the rate of younger women.

The reseachers suggested the decline could be attributed to the higher "socioeconomic status" of older women having babies in recent years.

In general, well-to-do mothers tend to get better prenatal care and hence are less at risk of complications.

Meanwhile, a Virginia genetic counselor says the belief that women are waiting longer to have babies is wrong. "The truth is that only professional women, who constitute a small percentage of all women, are waiting longer to become pregnant," said Dr. Thaddeus E. Kelly. "Women at large have actually begun having children at an earlier age over the past few years."

In 1950, up to 15 percent of U.S. women became pregnant after age 35, Kelly said. Now, only 5 percent do.