Black, Hispanic and Asian women - and a handful of white women not born in the U.S. - accounted for 41 percent of all births nationwide between 1993 and 2006, but for 62 percent of pregnancy-related deaths, researchers found.
"Everyone thinks the U.S. has great healthcare, which is definitely the case, but certainly there are populations in the U.S. that have outcomes that really look like outcomes in developing countries," said Dr. Allison Bryant, a maternal fetal medicine specialist from Massachusetts General Hospital in Boston who wasn't involved in the new study.
She said some of those differences may be due to minority women's lack of access to good pregnancy care - but that health problems growing up and before they become pregnant are likely to blame as well.
"If women are dying in the context of pregnancy, it speaks to a much broader problem in terms of disparities in medicine," Bryant told Reuters Health.
"If everyone had access to good pre-conception care... that might help to mitigate some of the things that we're seeing on the back end in terms of pregnancy."
Dr. Andreea Creanga and her colleagues used data collected by the CDC on all women who died during or within a year after pregnancy as well as birth records for their babies.
They found that for every 100,000 babies born to white women, between seven and nine moms died from complications related to pregnancy - including heart problems, infections and bleeding.
In comparison, 32 to 35 black women died for every 100,000 live babies. Deaths among Hispanic and Asian women - born in the U.S. and abroad - were closer to rates for white women at around 10 per 100,000.
Overall, older women were most likely to die during childbirth. Eighteen white mothers ages 35 and up died for every 100,000 babies born, as did 99 older black women - almost one for every 1,000 babies.
Consistent with recent research, Creanga's team also found a general increase in the number of pregnancy-related deaths, from 11 per 100,000 births in 1993 to almost 16 per 100,000 in 2006. It's not clear whether that rise reflects more women actually dying or just better identification of the women who do die from pregnancy-related complications, the researchers reported in Obstetrics & Gynecology.
They didn't have access to clinical details such as how babies were delivered and mothers' weight, both of which could affect pregnancy risks.
A total of about 7,500 women died of pregnancy-related complications during the 14 year-period.
Heart problems and general cardiovascular disease, including high blood pressure, were the most common causes of death in childbirth for both white and black women during the latter half of the study period.
"Similar changes have been observed in other countries, and very likely are due to combination of factors, including postponement of pregnancy and higher proportions of obesity, diabetes and hypertension among pregnant women," Creanga told Reuters Health.
Bryant said it's possible some differences in heart-related deaths are due to genetic or dietary differences between women. Deaths related to bleeding during childbirth, however, should be preventable with good medical care, she added.
More research will be needed to see what's really driving these disparities - and to figure out how to start addressing them, according to Bryant.
The study is published in journal Obstetrics & Gynecology.