Americans Are Having the Most Babies in These 20 Cities (BusinessWeek)

More babies = more spending

Where do American babies come from? Mostly west of the Mississippi, it turns out.

The map below shows U.S. cities whose populations had the highest share of babies in their buggies as of July 2014, based on Bloomberg calculations using Census Bureau data. Cities in Utah, Texas and California took the top eight spots, followed by Wichita, Kansas, and the metropolitan area that includes Omaha, Nebraska.

The common thread that unites many of these cities is that they have high numbers of young households, according to Mark Mather, associate vice president for domestic programs at the Population Reference Bureau in Washington. Young populations tend to have lots of babies because there are so many people of reproductive age, even if an area's fertility rate isn't particularly high, Mather said.

In Utah, "you have a young population combined with a high fertility rate," he said. 

That's especially the case for Provo, home to Brigham Young University, where many of the students are married with children, said John Curtis, who has been the city's mayor for more than five years. 

Additionally, "there's a high Mormon population," said Curtis, who has six children himself. "Mormons like big families — we're not bashful about that at all."

Utah also has a strong economy going for it, evidenced by an unemployment rate of 3.5 percent, compared with a level of 5.3 percent for the broader U.S.

"Wherever you've got job growth, you're going to have population growth, which often means you're going to have young families having lots of babies, and that creates population momentum," Mather said. Same goes for Texas, he said, where the unemployment rate is 4.2 percent.

And babies don't come cheap. A middle-income family who had a child born in 2013 can expect to spend about $245,340 — or $304,480, adjusted for projected inflation — for food, housing, child care and education and other associated costs up to age 18, according to the Department of Agriculture.

Those types of expenses can mean gaining population is "generally seen as a positive in terms of the economy," Mather said. Young families become bigger consumers, and their babies grow up to become spenders and taxpayers as well should they stay in the area. A growing citizenry could translate into political gains as well, in the form of extra congressional seats.

As with all things, there are some drawbacks to rapid population growth. It usually means heavier traffic, more crowded schools, higher child care costs and potential negative effects on an area's environment.

And there are limits to growth, "both in terms of the number of jobs available and in terms of space and natural resources," Mather said. "You need to keep fueling job growth in order to keep people moving in. People will leave if there are no jobs."

Methodology: Bloomberg ranked the 100 most populous U.S. metropolitan areas by birth rates, which were calculated as the number of births between July 1, 2013, to July 1, 2014 per 1,000 people in that area's average population for 2013 and 2014. The top 20 are shown in the map. Birth rates were ranked based on the second decimal point and rounded to one decimal point for the map.