This Mother’s Day, odds are that the new mother pushing the stroller has a few more years on her than in years past.

A federal survey being released Friday finds that the percentage of first-time mothers 35 or older has risen more than fivefold since the early 1970s, a trend toward “midlife moms” that demographers and social scientists say is having pronounced effects on the size, composition and future growth of the U.S. population.

The numbers are even more pronounced for the subset of women having their first child after 40. In 1970, an average of 0.4 women out of every 1,000 women waited until their 40s to have a first baby. By 2012, the average jumped to 2.3 women.

The trend toward older motherhood has pros and cons — medically, politically and socially — analysts say.

Births to women at older ages, especially at 40 and beyond, are associated with higher health risks for the infants and the mothers, although medical professionals say treatments and outcomes have improved in recent years.

On the positive side, older mothers are likely to have higher educations and higher incomes than their younger counterparts, as well as more outside resources.

Hollywood is following the trend. First-time mothers older than 40 include actress Halle Berry, model Cheryl Tiegs and singer Mariah Carey.

The consequences

Delayed childbearing has national implications because it “changes the population structure,” said T.J. Mathews, a co-author of the report from the National Center for Health Statistics.

When women give birth later in life, he said, they are less likely to have multiple children. Moreover, the timing of births has a ripple effect in the size and age of the national workforce and the elder population.

When paired with a falling teen mother birthrate over the same period, the survey finds that the average age for all first-time mothers in the United States has crept up by nearly five years in barely more than two generations. Women on average are 25.8 years old when they first give birth, compared with 21 in 1970.

The upward trend of first births to older mothers has been almost steady, Mr. Mathews said.

For women in their late 30s, “there was one drop for a few years,” from 2008 to 2010, but the rate started to increase again in 2011 and 2012, he said. Rates for women in their early 40s have risen slowly but steadily since 1985.

Put another way, in 1970, the federal government reported 14,146 births to women older than 35. In 2012, that number was around 133,000.

The reasons for older motherhood are not mysterious: Women pursuing higher education and careers tend to delay marriage and childbirth.

“Remember that we’ve seen postponement of many markers of adulthood in recent years,” said D’Vera Cohn, senior writer at Pew Research Center and co-author of a 2010 report, “The New Demography of American Motherhood.”

“In general, people are taking longer to graduate from college, they’re taking longer to get married, and they’re taking longer to have children,” she said.

Some of these factors are interrelated: “If you’re trying to finish your education, you may not want to get married and have a child until you’re done with that and established in a career,” Ms. Cohn said, noting that other Pew reports have shown that older mothers are likely to be college-educated.

Social acceptance

It is also socially acceptable to be an older mother, the 2010 Pew report found.

When some 1,000 adults were asked whether the trend of more women having babies in their 40s was good or bad for society, 47 percent said it made “no difference.” Another 13 percent said it was “good,” and 33 percent said it was “bad.”

Another major factor in older motherhood is the arrival of artificial reproductive technology.

Since 1978 and the birth of Louise Brown, the first “test-tube baby,” women in their 30s, 40s and even 50s have been able to give birth with artificial reproductive technology. Today, many, if not most, pregnancies after age 40 involve such technology because of a diminishing amount of healthy eggs and “poor embryo quality,” fertility researchers say.

Still, as absolute numbers show, older women are still greatly outnumbered by their younger sisters in maternity wards.

In 2012, there were 1.57 million first births, with more than 1 million born to women younger than 30, NCHS data say. Nearly 107,000 of these 2012 firstborns were to women ages 35 to 39, and 24,251 were to women ages 40 to 44. Moreover, 1,952 firstborns were reported for women in their late 40s, and 167 firstborns were delivered to women ages 50 to 54.

Of 1.61 million first births almost 20 years earlier, 76,129 were to women ages 35 to 39, 11,806 were to women in their early 40s and 425 were to women ages 45 to 49. Data were not listed for first births for women in their 50s in 1993.

“Anecdotally, if you are in that [older] age category and you had a first birth, and you know somebody else” who had also had a first birth at that age, “then maybe you might think, ‘Oh, everybody’s doing this.’ But the numbers are still pretty small,” Mr. Mathews said.

Among other highlights of the NCHS report, “First Births to Older Women Continue to Rise”:

• Asian-American women are consistently the most likely to delay their first childbirth. American Indian women are least likely to give birth for the first time after age 35.

• The District of Columbia has the highest rates of firstborn births to older mothers. Its 2012 rates are 29.9 births per 1,000 women ages 35 to 39 and 8.9 births per 1,000 women ages 40 to 44. (In 2012, the national average for the respective age groups were 11 births and 2.3 births per 1,000.)

• States with the highest firstborn rates to older women are Hawaii, Massachusetts, New Jersey and New York.

• States with the lowest firstborn rates to older women are Alabama, Arkansas, Mississippi, Oklahoma, South Dakota and West Virginia.

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