California birth rates are at historically low levels, and Tuolumne and Calaveras counties' rates are below the state average, according to 2010 figures compiled by the nonprofit Lucile Packard Foundation for Children's Health.
The organization compiled the data from reports released by the state Department of Public Health, state Department of Finance and the federal Centers for Disease Control and Prevention.
The state average is 63.0 births per 1,000 women between the ages of 15 and 44. Calaveras County had the third-lowest fertility rate in the state, with 47.0
Tuolumne County came in below the state average, with 58.6, but significantly outpaced most counties in the Sierra Nevada region.
Seven of the 10 lowest county fertility rates in California belonged to such mountain counties, with Amador (49.9) and Mariposa (50.9) counties the eighth- and 10th-lowest respectively.
The lowest rate in the state belonged to Sierra County at 39.9.
Demographers note that birth rates are typically tied to economic conditions and the national rate has fallen since the recession began in 2007. California has been slower than many other states to recover and continues to have higher unemployment rates than the rest of the country.
Rural counties in the Sierra foothills, in turn, have had unemployment rates stubbornly stuck in the double digits while the state rate has fallen below 10 percent.
"There's so many different factors, economic factors, employment, cost of housing … that impact whether young families are going to come to a community," said Tuolumne County Public Health Officer Dr. Todd Stolp.
Calaveras County's public health officer, Dr. Dean Kelaita, could not be reached for comment Thursday. However, Calaveras County was on the list of the 20 most economically stressed counties in the country as of February 2011, according to an index of unemployment, foreclosure and bankruptcy compiled by the Associated Press.
Stolp said, however, that he couldn't begin to speculate why Tuolumne County's fertility rate is quite high when compared to most foothill neighbors.
Another demographic consideration for the foothill region is a below-average Hispanic population for California, whose Latino population has the highest birth rates of any race or ethnicity. While 38 percent of Californians identified themselves as Hispanic in the 2010 U.S. Census, just 10.7 percent of Tuolumne County residents and 10.3 percent of Calaveras County residents did so. By comparison, Imperial County, on the Mexican border, which notched the highest birth rate of 81.7, had a Hispanic population of 80.4 percent of its residents.
San Francisco came in with the fourth-lowest birth rate of 47.6, an anomaly surrounded by rural counties in the bottom 10. A San Francisco Chronicle report tied that phenomenon to the city and county's small population of Hispanic residents (15 percent) and young families moving to the suburbs to raise their children where cost-of-living is more manageable.