Mirroring a trend that has led to the country's lowest ever recorded fertility rates, births to women in Sonoma County, particularly foreign-born women, have steadily declined since the recession began in 2007.

The rate for foreign-born women declined by about 20 percent from 2007 to 2010, compared to just under 6 percent for all women, according to data compiled by public health division of the Sonoma County Department of Health Services.

In Sonoma County, the rate for women who were born in Mexico declined by about 9 percent. Fertility rates are defined as the number of births per 1,000 females between 15 and 44 years of age.

A new study released this week by the Pew Research Center shows that across the country, dramatic birth rate declines have been driven by fewer births to immigrant women.

Peter Rumble, director of health policy for the Sonoma County Department of Health Services, said it's unclear what is causing the local decline.

"The interesting implication for local governments is the need to ensure we are responsive to and reflective of the community we serve," Rumble said.

"That is particularly true in areas with other demographic trends going on, like in Sonoma County where we have a strong and growing Latino population," he said.

Pew researchers said these birth rate declines are a result of behavior, not a change in the composition of the female population. There was no decline in the number of foreign-born women of childbearing age, despite a recent decline in illegal immigration from Mexico, which is the largest "source country" for immigrants to the United States, Pew said.

Nationally, the decline in births to foreign-born women was 14 percent, while Mexican-born women across the United States showed a dramatic 23 percent decline.

Also, the share of local babies of foreign-born women has declined since 2007, even as the share of babies born to U.S.-born women is rising.

Births to U.S.-born women comprised almost 64 percent of all births in 2010, up from nearly 59 percent in 2007. During that same period, the share of babies born to Mexican-born women dropped from 33 percent to just over 28 percent.

This trend is also reflected nationally, the Pew study said.

Pew, a nonpartisan research center in Washington, pointed out that its previous research found that the recent fertility decline may be linked to economic hardships.

Foreign-born and U.S-born Latinas showed bigger birth rate declines between 2007 and 2010, said Pew, adding that Latinos experienced a larger percentage decline in household wealth during the recession.