American women are waiting longer than ever to become mothers

More and more women in the United States are waiting until they're older to start having children.

The average age of women having their first child was a record high of 26 years old in 2013, according to the Centers for Disease Control and Prevention's National Vital Statistics Report.

That's an increase of 3.3 years since 1980, when the average age was 22.7.

Most of the change took place between 1980 and 2000, when the average rose to 24.9. It stayed relatively stable around 25 until 2008, when it started climbing again to its current high:

Business Insider / data from CDC

The average age of first-time mothers is increasing because more women are waiting until their 30s and 40s to start having kids and fewer women are having their first kids in their teens and 20s, the CDC report says.

In 1970, only 1.7 out of 1,000 women were having their first child between the ages of 35-39 years. By 2006, that rate had increased six-fold to 10.9 women out of 1,000. The rate took a little dip from 2006 to 2010 (corresponding to the dip in average age of first-time mothers), then made up the ground it lost: In 2012, 11.0 out of 1,000 women were having their first child between the ages of 35-39.

Since 1985, the rate of women having their first child between age 40-44 has risen steadily, though it's still far below that for women ages 35-39:

CDC

Rates for all births, not just of a first child, to women over 35 have been rising over the past 20 years, while birth rates for younger women are stable or declining. Still, the majority of births are to women under 35, about 85% of the total in 2013.

The CDC report says the increase in births to women over 35 has been linked to the use of fertility treatments and therapies, which can mean anything from talking to a doctor to doing in-vitro fertilization.

That makes sense, because a woman's fertility declines with age as her eggs accumulate abnormalities that make them less likely to develop into healthy babies if fertilized. But with more technologies on the horizon that might eventually offer older women increased chances of getting pregnant, it's possible the average age of women becoming mother for the first time will just keep going up.