Instead, the increase in recent years has been driven by heart problems and other chronic medical conditions, like diabetes, which has increased sharply in the population. Researchers have theorized that an increase in obesity — particularly acute among poor black women, who have much higher rates of maternal mortality than whites — may be contributing to the problem.

“The really scary thing to us is all the deaths from cardiovascular disease and heart failure,” said Dr. William Callaghan, who runs the Maternal and Infant Health Branch in the Division of Reproductive Health at the Centers for Disease Control and Prevention. “It’s a quarter of all deaths. There were almost none in the remote past.”

Maternal deaths are notoriously hard to count. There is often not enough detail on a death certificate to tell if the death was related to pregnancy. For example, if a woman dies from heart failure six months after she gives birth, it can sometimes take a special analysis to determine if it was pregnancy related (deaths can be counted up to a year after birth, though the vast majority happen in the first six weeks). In 2003, the federal government asked states to report in the same way, and most eventually complied.

Some have argued that the United States simply keeps better track now, counting deaths that would not have been included before. But federal health officials say the increase is more than just accounting.

“The rise is real,” Dr. Callaghan said.

Maternal mortality was relatively flat in the 1980s and 1990s, and most experts agree that the increases began around 2000.

The trend has puzzled researchers and prompted a number of states to start maternal death review boards, groups of experts who sift through the deaths and consider policy changes that might reduce them. Such boards, used in Australia, Britain and a number of other European countries, are considered crucial in understanding, and potentially reversing, the trend. But only about half the states have them.

“The first time I saw our results for the United States, I thought there must be some error,” said Dr. Nicholas J. Kassebaum, an assistant professor of anesthesiology and pain medicine at Seattle Children’s Hospital, who is the director of maternal and child health research at the Institute of Health Metrics and Evaluation. “I actually started looking for what went wrong in the data processing.”