Cities are noticing a new millennial trend: they are leaving. Urban planners are looking for a way to keep a generation moving toward marriage and childrearing in the city, according to a Washington Post report.

Many cities built high-rise apartment and condo buildings that were perfect for young, professional millennials, but now the oldest members of the generation are entering their mid-30s. People of this age are looking for more space, and cities are seeing a large exodus of millennials as a result.

But, urban planners are not going to sit back and watch the group that played such a big role in revitalizing cities leave them behind.

“We're trying to figure out what will drive this younger generation. Will they follow the same patterns of their predecessors, or will they do something different?" said Fred Selden, the planning director of Fairfax County in Northern Virginia, according to the report.

Many cities are looking to develop types of homes that have previously fallen out of style. Urban planners are seeing a renewed interest in duplexes, triplexes, row houses, and residential buildings with just a few units, according to the report.

Millennials aren’t necessarily looking for the white picket fence and the perfect yard, but they are looking for more space and more privacy, according to the report. In fact, they like much of what the city has to offer including proximity to public transportation.

Many millennials do not want to follow in the previous generation’s footsteps with a move to the suburbs, but the price of housing and issues like space in the city are driving many people to do so anyways.

It has yet to be seen if urban planners and developers will find a way to keep aging millennials in the city.