(CNN) -- The number of women dying each year due to pregnancy or childbirth in the United States has remained steady and some women remain more at risk of death than others, according to a new government report.
In 2018, the year with the most recent national data, a total of 658 women in the United States died while pregnant or within 42 days of the end of pregnancy, according to new data published in the US Centers for Disease Control and Prevention's National Vital Statistics Reports released on Thursday.
Maternal death was defined as the death of a woman while pregnant or within 42 days of being pregnant, from any cause related to or aggravated by the pregnancy or the management of the pregnancy. These maternal deaths in the new report do not include women who died by suicide or homicide.
In 2018, there were 17.4 maternal deaths per 100,000 live births in the United States, according to the report. When that number was broken down by race and age, significant disparities emerged.
The maternal death rate among black women was 37.1 deaths per 100,000 live births, a rate up to three times the rates for non-Hispanic white and Hispanic women, the report said.
The data also showed the maternal death rate among women 40 and older was 81.9 deaths per 100,000 live births, almost eight times the number for women under 25.
"These new data from our colleagues at CDC represent a landmark for this really important topic," said Dr. Janine Clayton, director of the Office of Research on Women's Health at the National Institutes of Health in Washington, DC, who was not involved in the new report.
"It gives us more evidence and more confidence that these maternal mortality rates that we're seeing in the United States -- that are still higher than those of our peer countries -- truly represent a critical public health issue and serious concern for the health of women," she said.
Clayton also addressed the racial disparity in maternal mortality numbers.
"We need research to understand why that gap persists and to be able to put in place measures to address factors that increase risk of mortality," Clayton said.
Researchers have known that the maternal death rate in the United States remains a public health concern and that about 60% of pregnancy-related deaths are preventable without much cost or trouble. And yet, the number has not gone down.
The number in the new report is similar to previous estimates. Separately, it has been previously estimated that about 700 women die each year nationwide as a result of pregnancy or childbirth complications, according to the CDC.
A shift in how maternal deaths are analyzed
For the new CDC report, researchers changed the method used to collect and examine data on maternal deaths using death certificates in the United States.
The new method was used in an effort to "mitigate errors in the pregnancy checkbox" system and "provide an improved estimate of maternal mortality," according to the report.
Beginning in 2003, states revised death certificates to include checkboxes to indicate whether a deceased woman was pregnant at or around her time of death. As of 2018, death certificates in all 50 states included some type of checkbox.
While analyzing death certificates, the researchers used their new method for coding maternal deaths. For that method, the application of the pregnancy checkbox was restricted to only people ages 10 to 44, compared with ages 10 to 54 in past years when states were adding the checkbox. Also, for those marked as pregnant only, the underlying cause of death was restricted to maternal-related causes, according to the new method.
"This is the first national estimate from NCHS that uses information collected from a pregnancy status checkbox in combination with the causes of death reported on death certificates," said Donna Hoyert, a health scientist in the mortality statistics branch of the National Center for Health Statistics in Hyattsville, Maryland, and an author of the new report.
"Although the addition of the checkbox was successful in identifying previously unreported maternal deaths, the checkbox was sometimes checked in error," she said. "The errors were concentrated with increasing age, so the 2018 method is designed to minimize the role of the checkbox without supporting cause-of-death information where research has found the checkbox to be less reliable."
In other words, "what NCHS has done is to say, 'OK we're not going to include any pregnancies over the age of 45 if the only reason we would be considering it is the pregnancy checkbox,'" said Dr. Elliott Main, a professor of obstetrics and gynecology at Stanford University School of Medicine and medical director of the California Maternal Quality Care Collaborative, who was not involved in the report.
"Meaning that there was nothing in the cause of death or the other descriptions on the death certificate that even mentioned pregnancy," he said. "That's a good start and that's the approach that they're using for 2018."
'658 deaths are 658 too many'
While the new method used in the report was intended to improve maternal mortality estimates, the researchers noted in the report that it "does not correct all errors, nor does it address all issues" and, due to the focus on females ages 10 to 44, it may result in underestimated maternal deaths at ages 45 and older, and overestimations for ages 10 to 44.
Studying maternal deaths, in general, can be challenging -- even "messy," Main said.
"Maternal deaths are a hard group to study because they are rare, roughly 1 to 2 per every 10,000 births," Main said.
"You would think that maternal mortality would be a very simple thing to measure -- a mother dies -- but it can be a bit tricky to determine whether a woman was pregnant or that the death was related to the pregnancy," he said. "It gets much more challenging when you're looking at causes that are not directly tied to the pregnancy, like blood clots or cardiovascular disease in particular, heart attacks, cardiomyopathy, and related complications."
Heart disease and stroke caused more than 1 in 3 pregnancy-related deaths between 2011 and 2015 in the United States, according to the CDC. Other leading causes of maternal death included infections and hemorrhage, which is severe bleeding.
The last time the NCHS published an official estimate of the US maternal mortality rate was more than a decade ago in 2007, which makes the new report "significant," said Dr. Michael Lu, dean of the School of Public Health at the University of California, Berkeley and former director of the federal Maternal and Child Health Bureau, who was not involved in the study.
The "most striking findings are the high overall mortality rate and large disparities -- nothing new, but still deeply troubling," Lu said.
"In 21st century America, in the richest and most powerful nation on Earth, no woman should ever die from pregnancy and childbirth: 658 deaths are 658 too many," he said. "I believe we can cut maternal mortality in half by 2025, and eradicate maternal deaths in the US by 2050, by doing three things: learn from every maternal death, assure quality and safety of maternity care for all women, and improve women's health across their life course."
The new NCHS data reflects "the scale of the public health challenge" the nation faces when it comes to maternal mortality, Dr. Maureen Phipps, CEO of the American College of Obstetricians and Gynecologists or ACOG, said in a written statement.
"Maternal mortality is a complex problem with disparities in outcomes stemming from many factors, including access to care, standardization of care, bias, and racism. Access to reliable, consistent data year after year is critical to establishing benchmarks, setting goals, and measuring progress towards improving outcomes," she said in part. "Continued improvement in tracking all data events -- including accounting for maternal deaths up to 12 months postpartum and deaths from suicide and substance use disorder -- is still needed."
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