As people weigh the costs of having a kid against what it takes to buy in these markets, some may decide to put off starting a family or to have fewer kids or none at all, Zillow said. People who can afford to live in an expensive area may also have jobs that make it harder to have kids before their 30s.

In a statement accompanying the report, Zillow Senior Economist Aaron Terrazas calmed statisticians everywhere by cautioning against drawing a cause-and-effect conclusion from the data. He added that one important question the research raises "is whether millennials are simply delaying childbirth into their 30s or are actually choosing to have fewer children."

For every 10 percentage points home prices rose, Zillow found a typical decline of 1.5 percentage points in birthrates beyond the drop it would expect to see if home prices were flat. The U.S. birthrate has been declining for years.

The trend was most pronounced in major California metropolitan areas and in Seattle; Austin, Texas; and Brooklyn, New York, where home prices spiked from 2010 to 2016, Zillow said. For example, in Los Angeles County, home values rose 31%, on trend with a 10% decline in the birthrate. Instead, Zillow found that the birthrate fell 17%.