The working mother has become the norm in America, with a new Census report that more than half of all new mothers are back in the job market within a year of giving birth.

The share of women returning to work, or actively seeking jobs, within a year after arrival of a baby climbed to 50.8 percent last year, the first time it has topped half of new mothers, according to the report "Fertility of American Women: June 1987."In 1976, when the Census Bureau first asked the question, only 31 percent or mothers went back to work within a year of a baby being born. It climbed to 49.8 percent by 1986.

"Every time a statistic approaches the 50 percent mark of the labor force, it's not an oddity anymore, its a way of life," Martin O'Connell, chief of the Bureau's Fertility Statistics Branch, said Wednesday.

The share of new mothers remaining in the work force has grown steadily as a result of women's increasing propensity to delay marriage and childbearing in favor of jobs and education, O'Connell said.

Women who accumulate more years of schooling and work experience before having children have "greater financial resources to enable them, once they have a child, to obtain child-care services and get back to the labor force much quicker," O'Connell explained.

In addition, he added, they have "increased career commitment and increased cost of staying out of the labor force, compared to an 18 or 19-year-old who who had a child earlier" in life.

For example, he pointed out that 68 percent of women who had their first child at age 30 or over remained in the labor force, compared to 54 percent of those aged 18 to 24.

The study also disclosed a substantial increase in the number of two-earner families with children. It concentrated on women aged 18 to 44, the normal childbearing ages.

The increasing share of mothers returning to jobs has led to a total of 13.4 million two-worker families with children, up from 8.3 million in 1976.

"A lot has been said about DINKS (dual-income, no kids families) ... given the attention, one would be led to think there are many more than there actually are," O'Connell said.

The number of childless couples with both husband and wife employed, with the wife still in the childbearing ages, increased from 3.0 million to 4.3 million" from 1976 to 1987.

"The real story is the number of dual-employed couples where they have a child . . . that's the big news that's been happening the last 10 years," said O'Connell.

At 13.4 million, dual-eaner couples with children make up more than 40 percent of all married couples in the childbearing ages, O'Connell pointed out.

That, he added, means that American businesses will have to pay close attention to the needs of these workers and families.

O'Connell addressed the topic last year in a separate, privately published, analysis - "Juggling Jobs and Babies: America's Child Care Challenge."

That report, which he said remains valid, warned that smaller numbers of people born in recent years means fewer workers available, making it harder for business to replace experienced women who have children.

The result will be a requirement to cope with the needs of working mothers, including such possibilities as flexible work schedules, employer-sponsored child care, flexible benefit plans and other new approaches, he wrote.

Other findings of the new Census study included:

-The national fertility rate for the 12 months ended in June 1987 was 71 births per 1,000 women aged 18 to 44, not significantly different from the rate of 70.3 a year earlier. The rate was 68.5 for white women, 83.2 for blacks and 95.8 for Hispanic women.

-The rate was 95.0 for married women, 38.1 for those who were single and 23.5 for women who were widowed or divorced.

-Labor force participation for new mothers was 53 percent for black women, 51 percent for whites and 36 percent for Hispanics.

-The labor force participation rate for new mothers with college degrees reached 63 percent, compared to 38 percent for those with just 12 years of schooling.