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Birth weight drops with income in U.S.

Health disparities are common in developed countries, including the United States, but at what age those inequities take root and how they vary between countries is less clear.

For a new study, researchers compared the link between income, education, and low birth weight in the United States with those in three comparable countries: the United Kingdom, Canada, and Australia. Low birth weight is a primary risk factor for infant deaths and is considered a key predictor of health and socioeconomic status throughout life and across generations.

The findings show that while low birth weight is linked to lower income and education levels in all four countries, that connection was most persistent in the US.

The research looked a low birth weights among babies born to mothers in five income groups and with varying levels of education. In the UK, Canada and Australia, disparities in birth weights were most pronounced between the highest- and lowest-income groups. But in the US, birth weights dropped consistently with income level. The findings underscore the degree to which inequity impacts health in the US, says lead author Melissa Martinson, assistant professor in the School of Social Work at the University of Washington.

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“We would expect in any country that there would be health differences between the highest incomes and lowest incomes, but what’s interesting in the US is how clear the distinction is for every dollar of income.”

The study, published in the American Journal of Public Health, looked at national birth weight data from maternal reports and birth certificates in the four countries, as well as maternal education and income data from longitudinal studies.

“It’s not just the very rich and poor whose health is tied to income in the US, but infants at every step of the socioeconomic ladder.”

The researchers controlled for differences in marital status, infant gender, and mothers’ race and ethnicity. They found that low birth weights—defined as 5.5 pounds or less— were highest overall in the UK (6.0 percent), followed by the US (5.8 percent), Canada (5.5 percent), and Australia (4.8 percent).

The data predates the Affordable Care Act, and the other three countries have more generous health care and social support systems than the US But low birth weights in the US are linked to factors beyond health insurance coverage, such as income instability, food insecurity, and residential segregation.

“If you’re a low-income woman and you grew up low-income and had poor nutrition and more stress, all these factors have accumulated throughout the life course to culminate in low birth weights,” she says.

Martinson began looking at health disparities across countries while employed as a social worker in the UK. Noting the differences in social services between England and the United States, she wondered what role those services played in health outcomes throughout life. Martinson published a paper in 2012 which found that health disparities by income were pervasive in both countries, despite England’s better overall health and universal health insurance.

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That prompted her to investigate whether inequities that contribute to health outcomes start at birth, and how those disparities might compare between the US and similar countries. The new study demonstrates that income and education matter more for health at birth in the US than in other countries.

“It’s not just the very rich and poor whose health is tied to income in the US, but infants at every step of the socioeconomic ladder,” Martinson says.

The Affordable Care Act could help mitigate low birth weight and other poverty-related health impacts, but consistent rates of low-birth weights in the US over the past half century—despite advances in reproductive technology, dramatic decreases in smoking rates, and expanded prenatal care — suggest that reversing the trend will take time.

“There are many questions about how health inequities emerge over the life course,” she says. “This research shows they’re there at birth. Whatever health disparities a woman has as a 50-year-old, they’re partially laid out for her at birth in the US, more than in these other countries.”

Nancy Reichman, a professor at the Robert Wood Johnson Medical School at Rutgers University coauthored the study.

Source: University of Washington