Birthrates fell to three-decade record low for the fourth time in a row last year, the Centers for Disease Control and Prevention (CDC) reports.

According to its latest figures, published Wednesday, there were 3.79 million live babies born last year.

That means that for every 1,000 women, 1,729.5 babies were born - several hundred shy of the birth rate needed to replace the population of the US.

The report comes just a day after the CDC revealed that deaths are on the rise among young Americans, increasing six percent for those under 65 as lief expectancy fell for the third year in a row.

Encouragingly, seven percent fewer teenagers are having children, but as more women focus on their careers and delay childbirth some experts worry the aging American population may not be replenished.

The rate (green) of live births in the US has been declining generally since 2009, falling 2% between 2017 and 2018 to reach a record low for the last 30 years for the fourth year in a row

As has been the direction of recent change, American women are waiting longer to start having children.

The average age of a first-time mother is now 26.9, up from 26.8.

Age at first childbirth went up nearly across the board, rising among Hispanic, white, Asian and black women.

It's a modest increase, but indicative of the larger trend.

While birthrates fell among women between the ages of 15 and 34 from 2017 to 22018, they increased among those aged 35-44 and held steady for very young women (10-14) and those 45 and over.

Teen birth rates are often harbingers of poverty for the new mothers and their children alike, so it's encouraging that these birth rates have continued to hit new record lows each year since 2009.

In 2018, 72 percent fewer teenagers had babies than did in 1991.

Meanwhile, birthrates among women in their 40s have risen by two percent.

'The rate for this group has risen almost continuously since 1985,' wrote the CDC report authors.

As the US population ages, fertility rates need to increase in order to replenish those who die.

But that population getting older also means women are aging out of their reproductive years, which end at the average age of 51 for American women.

However, the progress of infertility treatments means that women are extending their reproductive years.

In 2018, 959 babies were born to women over 50, a considerable 14 percent increase over 2017.

Childbirth remains dangerous in the US, where maternal mortality more than doubled between 1991 and 2014, but women are becoming more aware of and involved in their own birth plans.

Teen births (gray) continued to decline, falling by 7% between 2017 and 2018, while 2% more women had babies in the 40s (black)

Perhaps as a result, Cesarean sections are trending downward. In 2009, C-sections accounted for 32.9 percent of live births, but by 2018, that rate had fallen to 31.9 percent.

Declining rates of smoking and tobacco use before and during pregnancy also suggest a trend toward healthier pregnancy.

Only six percent of women reported smoking while pregnant in 2018, a decrease of nearly 8 percent since 2017.

While individual pregnancies may be getting healthier, this doesn't solve the population replacement problem in the US.

Men's fertility is suffering too, with men's average sperm concentration falling a baffling 52 percent in Western countries between 1973 and 2011, according to recent research from The American Society for Reproductive Medicine.

In order to create as many future mothers and fathers as the US has now, each woman needs to have two babies.

And currently, the birth rate is nearly 18 percent short of the 2,100 live births per 1,000 women needed to do that.