WEDNESDAY, Aug. 12 (HealthDay News) -- New mothers are getting older.

In the United States, the average age of women giving birth for the first time rose from 21.4 years in 1970 to 25 in 2006, according to a new report from the National Center for Health Statistics (NCHS).

Births to older women are partly responsible for the upward trend.

"In 1970, just 1 percent, or one in 100, [of] births were to women 35 and over," said study author T.J. Mathews, a demographer with the NCHS, which is part of the U.S. Centers for Disease Control and Prevention. "In 2006, it was one in 12 births. It's a dramatic transition."

On the other hand, the United States still has the youngest age of first-time mothers in the developed countries studied. In Britain, Switzerland and other nations, the average woman now has her first baby at nearly 30 years of age.

"The U.S. is only now caught up to where other countries were in 1970 [for average age of first-time mothers]," Mathews said.

Despite the surge in older moms, a high, but improving, teen birth rate keeps the U.S. figures down.

"We still do have a good number of unplanned and teen pregnancies," said Dr. Jennifer Wu, an obstetrician/gynecologist with Lenox Hill Hospital in New York City.

This new data is somewhat different from past analyses.

"We hadn't really looked at the component of average age of first births," Mathews said. "In the past, we looked at all births."

And first births are an important gauge of future trends, such as how many children a woman has, which affects population, as well as birth weight and birth defects.

The trend toward later motherhood was seen in all racial and ethnic groups and in all states plus the District of Columbia, but some areas saw larger gains than others.

The biggest jumps were 5.5 years in DC, 5.2 years in Massachusetts and 5.1 years in New Hampshire.

New Mexico, Mississippi and Oklahoma had the smallest increases: 2 years, 2.3 years and 2.4 years, respectively.

Three-and-a-half decades ago, Arkansas had the youngest average age (20.2 years) while the eastern states of Connecticut, Massachusetts and New York had the highest (22.5 years). In 2006, Massachusetts' first-time moms were the oldest (27.7) and Mississippi's the youngest (22.6).

Among racial/ethnic groups in the United States, Asian and Pacific Islander women now have the highest average age at 28.5 years, while American Indian/Alaska Native women have the youngest at 21.9 years.

The average age for first births in non-Hispanic white women is 26 years, 22.7 years for non-Hispanic black women and 23.1 years for Hispanic women.

The upward slope was most pronounced in the 1970s and 1980s and seems to be leveling off with differences between 2006 and 2007 appearing minor.

Some reasons for delaying starting a family might include younger women choosing to focus on advanced education and careers earlier in their lives, Wu said.

More information

Visit the National Center for Health Statistics for more on this and other health topics.