One-third of the estimated 3.7 million births in the year ended in June 1988 were to women in their 30s, a dramatic increase from the 19 percent in 1976, the Census Bureau said.

At the same time, the bureau, in its annual survey of the fertility of American women, said the number of premaritally conceived first births has risen sharply over the past 15 years and 22 percent of women between 18 and 44 who had a child in the previous year were not married.Overall, the Census Bureau said the fertility rate for the 53 million women between 18 and 44 years old was 69.7 births per 1,000 women. Hispanic women had the highest fertility rate - 94 births per 1,000 women - and accounted for 11 percent of all births in the survey period although they make up only 8 percent of all women between 18 and 44.

"Fertility rates during the 1980s have been stable and have averaged about 70 births per 1,000 during most of the decade," the report said.

In the 12 years since the bureau first began publishing annual fertility data, it said a clear pattern can be seen of women postponing childbearing to their early 30s.

In 1988, the fertility rate for women 30 to 34 was 81.6 births per 1,000 women, significantly higher than the rates in 1985 (69.9), 1980 (60), and 1976 (56.4). "The fertility rate for women 35 to 39 years old in 1988 (33.8 births per 1,000 women) was also significantly above both the 1985 (25.9) and 1980 (26.9) and 50 percent higher in 1988 than 1976 (22.6)," the report said.