A new study suggests that trends in home prices are correlated with birth rates in the US. Researchers say that counties with the highest home value growth have seen the biggest birth rate declines.

The findings suggest that in some cities, housing is too expensive to financially support children.

The trend could contribute to a possible " demographic time bomb."

No matter where they live, millennial women are generally having children later in life compared to past generations.

A rise in home prices affects how a community functions. It can change a neighborhood's storefronts, tax base, and even food prices.

Another result of expensive housing may be that residents have fewer babies. According to a new study from real-estate site Zillow, birth rates have dropped the most in cities that have seen the largest growth in home values since 2010.

Sarah Mikhitarian, a senior economist at Zillow, says this may not be a coincidence.

"Past research has examined how rising housing costs are contributing to delays in a handful of key life milestones, like getting married and purchasing a first home — both of which are common steps toward starting a family," she told Business Insider. "And many people strive for financial stability before becoming parents."

After calculating median home value growth percentages across 214 US counties from 2010 to 2016, Zillow grouped them into four ranges. Counties that saw their median home price decline from 2% to 28% were considered to have the least home value growth. Counties where home prices increased anywhere from 30% to 66% were labeled as places with the highest home value growth.

The researchers then compared those figures against the CDC's fertility data for women ages 25 to 29 over the same period. The chart below shows a correlation:

San Francisco's median home price increased by 61% from 2010 to 2016, while the city's fertility rate fell by 22% over the same period. In Davis County, Utah, home prices rose by a mere 4%, and birth rates decreased by just 8%. East Baton Rouge, Lousiana's median home price decreased by 3%, and fertility rates actually rose by 9%.

Mikhitarian admits the study has a few caveats. For one, an area's rising home prices may have nothing to do with its declining fertility rate. Young people who settle in cities are generally less likely to have children compared to those who live outside urban areas.

In addition, the researchers only considered the fertility rates of women in their late 20s — a limitation since millennial women are having fewer children than past generations.

In 2017, the US birth rate hit an all-time low, according to the CDC. Women in the US gave birth to around 3,853,472 babies last year — a 5% drop from 2000. Until 2008, the national fertility rate for women in their 20s was around replacement level (the rate at which a generation can replicate itself). But it's been declining ever since.

The trend of women in cities having fewer babies could contribute to something economists call a "demographic time bomb." When there are not enough young people entering the workforce to replace older workers and pay into social security — at the same time that longevity increases — that can shrink the economy over time.

Social scientists attribute this multi-decade trend to an overall growth in women's access to birth control and sex education, an increase in education and career opportunities for women, and shifting societal ideals around gender roles.

That said, the data may highlight a somewhat unexpected way the nation's affordable housing crisis is affecting the millennial generation. If a woman in her 20s chooses to buy a home in an expensive city, she may wait to procreate until she can afford diapers, pediatrician visits, and a crib. Or she may decide to forgo biological children altogether.