Americans aren't having enough kids. That's bad news for the economy — and immigration may be the answer.

• The US population is expected to peak later this century. That's bad news for the economy.
• China, Japan, and Italy are struggling to address low birth rates.
• Economists say immigration is one way to keep the population rising.

Some of the world’s largest economies are grappling with a new problem for the first time.

China, Japan, and Italy are all battling dwindling birth rates — and their struggles are a reminder of an issue that could eventually become a problem for the US. The issue meant China last year lost its crown as the world's most populous country to India — a record it'd held for almost 75 years, according to the United Nations.