While the causes of congenital heart defects are often unclear, a new California study shows that poverty and pollution are risk factors.

The findings bolster previous research and have implications for Philadelphia, the nation’s poorest big city, where many neighborhoods carry the toxic legacy of their industrial pasts.

Preliminary data from the California study will be presented Monday at the American Heart Association’s annual meeting, which began at the Convention Center on Friday.

Congenital heart defects — the most common birth defects — are structural abnormalities that arise in the heart or nearby blood vessels as a fetus is developing. Among newborns, the incidence of these defects has been reported in studies to range between 4 and 10 per 1,000 births.

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Although some abnormalities are minor and others can be surgically corrected, an estimated 1.3 million Americans are living with chronic cardiovascular problems stemming from the defects.

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The new study, led by scientists from the University of California, San Francisco, mined a large California population database to get six years of health and demographic data for more than 2.4 million newborns and their mothers. The overall incidence of serious heart defects was relatively low — 3.2 per 1,000 births.

The team used census and state data to drill down to the neighborhood level, assessing socioeconomics — occupation, education, and wealth — and exposure to pollutants.

In the poorest neighborhoods with the worst environmental pollution — where 10% of the state’s residents live — the odds of a baby being born with a heart defect were almost 40% higher than in the wealthiest, cleanest neighborhoods. Even in the least polluted neighborhoods, low socioeconomic status was linked to about a 23% increase in heart birth defects.

The study found only an association. It was not designed to determine exactly how neighborhood conditions acted on pregnant women to produce a higher risk of birth defects.

Genetics plays a role in congenital heart defects, but so do mothers’ health problems and habits. Previous studies have found that diabetes, hypertension, smoking, and drinking during pregnancy increase the risk of such defects. Economically disadvantaged children are also at higher risk of dying of problems related to heart birth defects, a study published earlier this year found.

The authors’ conclusion noted that pollution and poverty “may precede other risk factors in the causal pathway to the development” of congenital heart defects “and may be a target for social policy initiatives.”

“Basically, it’s not social deprivation itself that increases the risk of congenital heart defects, but other factors that occur as a result of social deprivation,” said lead researcher Shabnam Peyvandi, a UCSF professor of pediatrics, epidemiology, and statistics.