The pregnancy mortality rate is dropping in most wealthy nations, but not here: The rate of women dying in childbirth is going up in the United States.

CDC - cdc.gov

The rate of pregnancy-related deaths more than doubled between 1987, the first year the Centers for Disease Control and Prevention began collecting data on pregnancy mortality, and 2012, the latest year for which data were available, the agency reported.

"Since the Pregnancy Mortality Surveillance System was implemented, the number of reported pregnancy-related deaths in the United States steadily increased from 7.2 deaths per 100,000 live births in 1987 to a high of 17.8 deaths per 100,000 live births in 2009 and 2011."

The overall risk of dying from complications during a pregnancy remains low, but the trend offers insight into the current state of public health in the United States.

Why are pregnancy-related deaths growing in the United States?

"We've seen a big bump in cardiovascular disease and chronic disease contributing to maternal deaths," William Callaghan, chief of maternal and infant health at the CDC, told Vox. "Underlying heart disease is common. Diabetes is common. We now have a group of women bringing with them into pregnancy their entire health history."

CDC - cdc.gov

Fewer women are dying from emergency pregnancy complications such as hemorrhaging, embolism, and pregnancy-induced hypertension, Vox reported. Rather, more women are dying from complications related to preexisting chronic diseases, particularly cardiovascular illness.

Cardiovascular disease is now the second-leading cause of pregnancy-related death in the U.S.; 30 years ago, it accounted for fewer than 10 percent of pregnancy-related deaths, Vox reported.

Common chronic health conditions, such as hypertension, diabetes, and obesity, contributed to the uptick in pregnancy mortality because they put women at a much greater risk for pregnancy complications, according to Vox. Callaghan explained the wider context of the trend:

"It's a larger problem than just dealing with women during pregnancy. It's the health of our society. Imagine a [pregnant] woman comes in with [a body mass index] of 40, and she's 24 years old. That didn’t happen in the past year; it happened in the past 24 years."

When you look closer, there's another disturbing trend revealing considerable racial differences.

Black women are much more likely to die during childbirth than women of other races and are more than three times more likely to die from pregnancy complications than white women, the CDC found.

The disparity is growing worse, Vox reported. Some potential explanations, with a caveat:

"Studies have shown that Black women are less likely to begin prenatal care in the first trimester and are more likely to have preexisting chronic conditions, such as hypertension, diabetes, or obesity, than white women. But this still doesn't account for the enormity of the disparity that currently exists."

The disparity may reflect a racial bias in other areas of health care. Black patients are more likely to die in hospitals because doctors typically express greater compassion in their body language toward white patients and are more distant with Black patients, research has shown. Doctors' poor nonverbal communication could have a huge negative effect on their treatment of Black patients.

Research has also shown that therapists aren't as likely to return calls from prospective Black and low-income patients, even though these groups are at higher risk for developing mental illness than their white and more economically stable peers.

The answer — for now — is more research.

Researchers need more reliable data to get a more accurate picture of maternal deaths, especially since one in three are preventable, Vox reported. Establishing more maternal death review boards across the country could help provide that much-needed data.