Researchers at the U.S. Centers for Disease Control and Prevention (CDC) found that between 1998 and 2005, the rate of pregnancy-related deaths was 14.5 per 100,000 live births. And while that rate is low, it is higher than what has been seen in the past few decades.

The researchers caution that the extent to which the rise reflects a true elevation in women's risk of dying is unclear. Recent changes in how causes of death are officially reported by states to the federal government may be at least partially responsible for the findings. However, it is also possible that part of the increase is "real." According to the new data, deaths from chronic medical conditions that are exacerbated by pregnancy, including heart disease, appear to account for a growing number of pregnancy-related deaths.

In contrast, deaths from actual obstetric complications -- namely, hemorrhaging and pregnancy-related high blood pressure disorders -- are declining. The absolute risk of a U.S. woman dying from pregnancy-related problems is still "very small," lead researcher Dr. Cynthia J. Berg, of the CDC's division of reproductive health, said in an interview.

But, she added, the new findings do underscore the importance of women "making sure they are in the best possible health before pregnancy."All women, Berg said, should try to have a pre-pregnancy visit with their ob-gyn and, if needed, get their weight and any chronic medical conditions, like high blood pressure or diabetes, under control before becoming pregnant.

For their study, Berg and her colleagues looked at data on 4,693 pregnancy-related deaths reported to the CDC between 1998 and 2005. Pregnancy-related death was any death occurring during or within one year of pregnancy that was attributed to a pregnancy complication.

The researchers estimate that for that eight-year period, the national rate of pregnancy-related death was 14.5 for every 100,000 live births.

In contrast, in 1979, there were just under 11 maternal deaths per 100,000 live births in the U.S. -- a rate that fell to as far as 7.4 per 100,000 in 1986, before beginning a gradual increase.

In addition, the racial gap that has long been seen in pregnancy-related deaths shows no signs of narrowing. Between 1998 and 2005, the death rate among black women was 37.5 per 100,000 live births, versus 10.2 per 100,000 among white women and 13.4 per 100,000 for all other racial groups combined. The reasons for the upward trend in the overall rate of pregnancy-related deaths are not certain, and more studies are needed to tease apart the contributing factors, Berg said.

One factor, according to the researchers, could be two technical changes in how causes of death are officially reported. In 1999, the U.S. adopted an updated system for coding causes of death -- one that allowed more deaths to be classified as "maternal." Then in 2003, the standard death certificate was revised to include a "pregnancy checkbox," which increased the number of deaths that could be linked, in timing, to pregnancy.