A new study has found that the link between low birth weight and lower income and education levels is greater in the U.S. than 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, according to lead author Melissa Martinson, Ph.D., a researcher at the University of Washington.
The study, published in the American Journal of Public Health, looked at low birth weights among babies born to mothers in five income groups and with varying levels of education.
The study found that in the U.K., Canada and Australia, disparities in birth weights were most pronounced between the highest- and lowest-income groups.
But in the U.S., birth weights dropped consistently with income level.
The findings underscore the degree to which inequity impacts health in the U.S., Martinson said.
“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 U.S. is how clear the distinction is for every dollar of income,” said Martinson, an assistant professor in the University of Washington School of Social Work.
The study 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.
The researchers noted they 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 U.K. (six percent), followed by the U.S. (5.8 percent), Canada (5.5 percent) and Australia (4.8 percent).
The researcher noted that the data predates the Affordable Care Act, and the other three countries have more generous health care and social support systems than the U.S.
But Martinson added that low birth weights in the U.S. 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 said.
Martinson began looking at health disparities across countries while employed as a social worker in the U.K. Noting the differences in social services between England and the United States, she wondered what role those services played in health outcomes throughout life.
She published a paper in 2012 that found that health disparities by income were pervasive in both countries, despite England’s better overall health and universal health insurance.
That prompted her to investigate whether inequities that contribute to health outcomes start at birth, and how those disparities might compare between the U.S. and similar countries.
The new study demonstrates that income and education matter more for health at birth in the U.S. than in other countries, she said.
“It’s not just the very rich and poor whose health is tied to income in the U.S., but infants at every step of the socioeconomic ladder,” she added.
Martinson said the Affordable Care Act could help mitigate low birth weight and other poverty-related health impacts. But she noted that consistent rates of low-birth weights in the U.S. 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 said. “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 U.S., more than in these other countries.”