Fertility numbers are in for the third quarter of 2016, and Forbes reported Friday that in the U.S. "general fertility has fallen, with younger age brackets showing the steepest declines."

A full year drop in 2015 matched 2013's all-time low, Forbes stated, and opinions are mixed over whether or not they will rebound. While the under-30 age group fell, the over-30 age group has risen, but not enough.

"The overall birthrate is gradually falling because the percentage drop in birthrates under age 30 is larger than the percentage rise in birthrates over age 30. Keep in mind that under-30 age brackets have always accounted for the most births, though the margin has been shrinking over time," the article maintained.

The net loss may be puzzling due to gains in the economy where Forbes said, "real median household income rose by 5.2% in 2015, the biggest rise ever recorded by Census and the first rise since the Great Recession." However, economic gains had not been "showing up in the birthrate."

Fertility demographers point to Millennials and their "lifestyle habits" that may be a cause for concern in future fertility rates. Not only is that population segment opting to delay parenthood, but Millennials have also shifted to more urban areas where rates tend to be lower.

Optimists in the arena maintain history has shown a "tempo effect" that was "the observed fact that fertility declines are often followed by fertility rebounds, since women who defer having babies for a few years (say, during times of recession or war) often make up for it by having more babies later."

Pessimists, on the other hand, predict "Millennials don't want to end up with families that are quite as large" as previous generations.

Add to that the impact of a shrinking immigrant birth rate.

"More broadly, according to a 2015 report by the Center for Immigration Studies, the birthrate for women age 15 to 50 declined more than twice as much for all immigrants as for all native-born Americans between 2008 and 2013. And looking ahead, immigration is poised to further drag down birthrates," Forbes reported.

What lies ahead may be a lower birth rate in the U.S., demographers charge.

"In 2016, forecasting firm Demographic Intelligence downgraded its earlier forecast and predicted that the U.S. TFR would not go back above 1.9 for the next five years or longer," the article stated.