Murder is a surprisingly common cause of death among pregnant women in the United States, U.S. government researchers reported on Wednesday.

Black women are especially vulnerable to being killed while pregnant, the team at the Centers for Disease Control and Prevention found.

“Homicide is a leading cause of pregnancy-associated injury deaths,” Jeani Chang and colleagues wrote in the latest issue of the American Journal of Public Health.

They investigated the deaths of women who died while pregnant or within a year of being pregnant between 1991 and 1999 and found 1,993 that were caused by injury, compared with 4,200 that were directly related to pregnancy complications.

Of the injury-related deaths, 617 or 31 percent were ruled homicide, making murder the second most common cause of injury-related death for pregnant women after car accidents.

The homicide rate for pregnant black women was more than triple that for white women, the researchers said.

Most of the murdered women, 56 percent, were shot to death while the rest were either stabbed or strangled.

On Tuesday, a Texas man was charged with murdering his pregnant lover and her 7-year-old son after he led police to a makeshift grave.

Stephen Barbee, 37, told police he murdered Lisa Underwood, 34, and her son Jayden because she was pregnant with his child and wanted him to leave his wife for her, according to court documents quoted by the Dallas Morning News.

A second report suggests that pregnancy-related deaths may be underreported.

Isabelle Horon of the Maryland Department of Health and Mental Hygiene collected data on maternal deaths from death certificates and other records.

She found 38 percent of the deaths in pregnant women or women who had recently given birth were not reported as being pregnancy-related.

“The number of maternal deaths is substantially underestimated when death certificates alone are used to identify deaths,” she wrote in her report in the same journal.

“For example, if a doctor writes on a death certificate that ‘hemorrhage’ was the cause of death, and doesn’t mention that the hemorrhage occurred postpartum, this death would not be counted as a maternal death,” she said in an e-mail.