More U.S. women are dying from pregnancy-related causes, and more than half of those deaths are preventable, government health officials said in a report Tuesday.

Although these deaths are rare — about 700 a year — they have been rising for decades, especially among black women.

“An American mom today is 50 percent more likely to die in childbirth than her own mother was,” said Dr. Neel Shah, a Harvard Medical School obstetrician.

The American College of Obstetricians and Gynecologists meanwhile has released new guidelines saying women should have a comprehensive heart-risk evaluation 12 weeks after delivery, but up to 40 percent of women don’t return for that visit and payment issues may be one reason.

Bleeding and infections used to cause most pregnancy-related deaths, but heart-related problems do now.

“Pregnancy is really a stress test” because of the extra blood the heart is moving for mom and child, said the head of the guidelines panel, Dr. James Martin. That can reveal previously unknown problems or lead to new ones.

Tuesday’s Centers for Disease Control and Prevention report found that about one third of maternal deaths happened during pregnancy, a third were during or within a week of birth, and the rest were up to a year later.