Nagi, Japan (CNN) – When Katsunori and Kaori Osaka had their first child, they were living in a cramped apartment in Nagoya, a city of more than 2 million people in central Japan.

Like many other young couples, they tried to raise their child in the city but found life among the apartment blocks too crowded and expensive, with few child care options. Eventually, they gave up.

"When people are in their 20s and 30s, they can't really afford to live in a bigger space in a city," Katsunori said. "We knew that if we wanted to have more kids, we couldn't do it there."...