Call it the Miracle of Dane County.

Or maybe the Mystery of Dane County.

Last year, health officials in the Wisconsin county noticed that infant mortality among blacks had dropped by a stunning 70 percent in a five-year period. In the rest of Wisconsin, the black infant mortality rate remained the highest in the nation. But Dane County, which encompasses the capital city of Madison, completely closed the gap between white and black, a gap that appears in virtually every community in the United States.

Wisconsin health officials cheered the news -- but couldn't explain how it happened.

A closer look at the data found that infant mortality among blacks dropped because they had many fewer extremely premature babies, and those who were born prematurely were recovering better.

The county health department is studying what happened and expects to have an answer in three years. The findings will be of interest around the country because the high rate of preterm births among blacks remains one of the great puzzles of the prematurity epidemic. Nationwide, 18.1 percent of black babies are born prematurely, compared with 11.5 percent of whites.

Pregnant women who are poorly educated and have little income or social support are more likely to deliver preterm babies. African-Americans are disproportionately represented among the poor, so socioeconomic factors have long been blamed. But then, studies showed that even high-income, highly educated black women were having more preemies. In fact, a college-educated black woman is still more likely to have a premature baby than a white woman who didn't graduate from high school.

Some believe that the racial disparity in preterm births may be determined by genes. Certainly genetics are known to play a role in premature births, which can run in families. But genes cannot explain it all. Otherwise, African-born immigrants to the United States would also have high preterm birth rates. Instead, at least according to one study, African immigrants had preterm birth rates similar to those of white Americans.

"Race is really not a biological concept. It's a social concept," says Dr. Diane Ashton, deputy medical director of the March of Dimes, which since 2003 has focused on understanding and fighting premature births and which has financed research and social services targeted at African-Americans.

A newly developing theory, Ashton says, points a finger at racism. This theory holds that day-by-day encounters with discrimination could promote premature birth, by inducing chronic stress. A 2004 study found a connection between an African-American woman's lifetime exposure to racism and her likelihood of delivering a very low birthweight child.

It could be as simple as "someone deciding as you walk into a store -- they don't see your degree -- but you are a potential thief," says Dr. Dannie Ritchie, director of the Transcultural Community Health Initiative of Brown University, and an African-American family doctor who has studied the effect of inequality on health.

"It's vigilance. It's being hypervigilant, always concerned that something is lurking to discredit you," Ritchie said.

One of the early observations from Dane County seems to back up that idea. The researchers talked to focus groups of African-American women of two generations, and were struck by the difference in their feelings. "The weight of discrimination and racism seemed to be fairly heavy in the older group of women," said Dr. Thomas Schlenker, Dane County's public health director. Younger women, in contrast, "seem to feel very positive about how they're accepted into the community."

The high quality of life in the county, and quality of health care, could also play a role.

"What I hope [the research] shows is that living in an inclusive and supportive community is good for you," Schlenker says. "And if we can demonstrate how that actually makes a difference in how babies survive or not … we will really have something to share with other cities in Wisconsin and elsewhere."

ffreyer@projo.com