Philadelphia ranks fifth from the bottom of the more than 200 most populous counties nationwide on a key measure of infant mortality, the organization Save the Children is reporting Tuesday, another reminder of the overriding importance of poverty and race.

Some other parts of the region don't fare well either. Camden County was No. 20 and Delaware County No. 22 in the analysis of infant deaths in the first 24 hours of life for 2007 through 2009.

Among states, Delaware comes in fourth and Pennsylvania sixth (counting the District of Columbia as No. 1).

Infant mortality has long been one of the most important barometers of societal health.

"It measures more than just babies dying," said city Health Commissioner Donald F. Schwarz, a pediatrician. "It measures health of women, access to care for women and children, the safety of children, the infectious disease [risk], the quality of water and sanitation."

As child mortality has plummeted in recent decades, the result of concerted efforts by governments and health workers, one statistic has proven harder to move: death on the first day of life.

One million babies a year die within 24 hours of birth, according to Save the Children, nearly all of them in the developing world. Among industrialized nations, however, the United States ranks last - 11,000 infants die on Day One, representing 35 percent of all child deaths by age 5.

In highlighting Day One, the group, whose U.S. headquarters is in Westport, Conn., is hoping to raise awareness and funding for what it says is an underrecognized problem that is distinct from the deaths of babies even a month old. Most first-day deaths everywhere are due to preterm births.

"One thing that women in Philadelphia have in common with women in the developing world is that the poorer they are, the greater chance they have of losing a baby on the first day of life," said Carolyn Miles, Save the Children's president and chief executive officer.

African American women in particular are two to three times more likely to give birth early, for reasons that are not clearly understood.

Many of the usual suspects - drug use, obesity, access to care - play relatively small roles, said Jennifer Culhane, who studies racial disparities in preterm birthrates at Children's Hospital of Philadelphia. "White women smoke in pregnancy way, way, way more than black women," she said.

Research increasingly suggests that race and poverty are independently related to preterm birth, with poor African American women at greater risk than poor white women.

"There is something about being disadvantaged both economically and socially marginalized - basically being black," Culhane said, "that is stressful on a day-to-day basis" and "can alter your physiological process such that women who are more stressed are more vulnerable to inflammation."

Much as stress at work can make it more difficult to fight off a common cold, constant high levels of stress make women more vulnerable to otherwise minor uterine infections that may lead to an early delivery, she said.

Philadelphia has the highest poverty rate and the highest percentage of African American residents of the nation's 10 biggest cities. But several experts agreed that the city's exceptionally poor showing on infant mortality - and relatively poor statistics for Pennsylvania, Delaware, and some suburban counties - may also be due in part to how the numbers are reported.

There is wide variation, for example, in whether infants born so prematurely that they will almost certainly not survive are counted as "live births," which can increase the infant mortality rate when they die. The United States, with all its lifesaving technology, is more likely to do so than developing countries, which may explain why first-day infant mortality in this country is similar to that of China and Egypt, and why Philadelphia is closer to Indonesia.

And federal health data is collected mainly by county. So it includes Philadelphia - which is both a city and a county - and a handful of other municipalities but not, say, Detroit, whose far-higher infant mortality numbers are reported as an average with surrounding Wayne County, Mich.

"But I don't want to skirt the real issue. We have a problem," said Jay Greenspan, a neonatologist who is pediatrics chair at Thomas Jefferson University.

A Philadelphia Medical Examiner's Office review of 487 deaths of children less than one year - nearly half of them in the first 24 hours - in 2009-10 attributed 54 percent to prematurity. An additional 18 percent were due to sleep-related causes, such as suffocation due to an adult rolling over in a shared bed; 11 percent were congenital.

When an infant is born prematurely, "none of the organs are ready for prime time," said Greenspan. "The lungs are not mature until 34 to 35 weeks. They are prone to bleeding everywhere." Bleeding in the brain can cause cerebral palsy. Infections due to an immature immune system can cause other problems.

Preterm birth also raises the risk of many more manageable conditions that have been increasing in recent years, such as autism, asthma, and attention deficit disorder.

Greenspan said that lack of access to prenatal care and delivery rooms is a problem in the city, where 15 maternity wards have closed in the last two decades. Six remain open.

The State of Delaware, where he is also pediatrics chief at Alfred I. du Pont Hospital for Children, has made a concerted effort over the last several years to improve infant mortality rates, which had been even worse.

The initiative, enshrined in state law, has focused on those factors that it could influence: family-planning awareness, antismoking campaigns, reducing elective cesarean sections, increasing access to prenatal care, and, in particular, providing a form of the hormone progesterone that can reduce the risk of early delivery.

Pennsylvania also has a few small programs targeting preterm births, including a year-old effort with Einstein Medical Center that regularly runs a group prenatal care clinic at Enon Baptist Church, said Amy Flaherty, who oversees the program for the state Department of Health.

Research suggests that even prenatal care is likely to make only a small difference in the complex issue of preterm births. But every bit helps.

"Even coming out one or two weeks early knocks points off your IQ," said Joy Lawn, a professor at the London School of Hygiene and Tropical Medicine, who analyzed the data for Save the Children.

Contact Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.