The housing markets that have seen the greatest increases in home values have also seen some of the most pronounced decreases in birth rates, according to an analysis by the real estate site Zillow.

The company recently looked at the changes in values and birth rates in several counties in the United States between 2010 and 2016. It concluded that a boost of 10 percentage points in home values was associated with a drop of 1.5 percentage points in the birth rate among 25- to 29-year-old women.

The effect was especially pronounced in California, where home prices have been rising rapidly in recent years. In Alameda County, in the Oakland area, home prices were up 60 percent between 2010 and 2016 while the birth rate among young women dropped by 24 percent. A 58 percent increase in home values in Santa Clara County, in the San Jose area, was associated with a 20 percent reduction in the birth rate.

San Diego County had a 34 percent increase in home values and a 19 percent drop in the birth rate, while the birth rate in Los Angeles County was down 17 percent as home values rose by 31 percent. In San Bernardino County, home values were up 49 percent as the birth rate among young women fell by 8 percent. Orange County had a 28 percent increase in values and a 16 percent decrease in the birth rate.

Outside of California, significant reductions in birth rates were observed in Texas, New York, and Washington. In Travis County, Texas, which includes Austin, home values increased by 33 percent and the birth rate was down by 22 percent. Home values in Seattle's wider metro area of King County rose by 32 percent as the birth rate fell by 18 percent. The birth rate in Kings County, N.Y., which includes Brooklyn, was down 15 percent, while home values were up 36 percent.

In Florida, significant price increases weren't associated with as significant a decline in birth rates. Values were up by 50 percent in Miami-Dade County, but the birth rate fell by just 4 percent. In Palm Beach County, home values rose by 37 percent and the birth rate fell by 4 percent.

Some markets saw a decrease in both home values and birth rates between 2010 and 2016. Value fell by 8 percent in Baltimore County, Md., while birth rates were down by 13 percent. In New Castle County, Del., which includes Wilmington, the birth rate dropped by 19 percent as home values fell 7 percent. Middlesex County, N.J., which includes the city of Edison, had a 7 percent reduction in values and a 21 percent drop in the birth rate among young women.

Zillow said the findings might be a reflection of younger people in more expensive markets putting off having children until they can move to a more affordable area or gain enough financial security to afford both a home and the costs associated with raising a child. However, the company also cautioned that numerous factors could influence whether a couple opts to delay having children or not have children at all, and that correlation in the figures does not necessarily indicate causation.

"The big question this research raises is whether millennials are simply delaying childbirth into their 30s, or are actually having fewer children – and so far the data are mixed," said Aaron Terrazas, senior economist at Zillow. "Recently released preliminary data shows fertility rates among 30-something women fell in 2017 for the first time since 2010, an ominous sign, but then again, respondents to recent census surveys indicate they expect to eventually have just as many children as respondents in prior years. Ultimately, this data adds another layer to the argument that rising housing costs are contributing to meaningful delays in achieving a number of key life milestones, including getting married and buying a first home – two very important steps on the road to starting a family."

A previous study by the U.S. Federal Reserve Board and University of Maryland determined that increases in home prices had a mixed effect on birth rates. Those researchers found that birth rates went up among homeowners whose property values increased, perhaps due to a couple realizing greater financial security from the added equity and going forward with their goal of raising a child. However, birth rates also fell among non-owners when home prices increased.