For decades, breast cancer has been less common in black women than white women, yet killed black women at a higher rate.

One of those gaps has finally closed. Unfortunately for black women, it's the first one.

A new report suggests black and white women are now being diagnosed with breast cancer at the same rate, because diagnoses have grown more common in black women while the white rate has leveled off.

Meanwhile, the death gap actually widened.

"It's a terrible situation" for black women, said the lead author of the new report, Carol DeSantis of the American Cancer Society.

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The article was published electronically Thursday in CA: A Cancer Journal for Clinicians.

The findings are based in part on historical data from cancer case registries in nine parts of the country. Together, the areas represent about 9 percent of U.S. population.

In 2012, the black and white rates converged at around 135 cases per 100,000 women, the researchers found. A decade earlier, the white rate was 132 and the black rate down at 124.

Experts think white women have had higher rates for several reasons. One is more white women waited longer in life until they have children, and science suggests earlier childbirth is associated with a lower risk of developing breast cancer later on.

But while the white rate leveled off, the black rate continued to creep up. Researchers don't exactly know why.

Stepped-up screening in some parts of the country may be a factor. But the researchers say the leading theory has to do with obesity.

In older women, being obese is tied to a higher risk of developing breast cancer. And obesity rates are higher in black women and have been rising more dramatically.

The researchers looked at other data and saw seven states where black women had higher rates of newly diagnosed breast cancer than white women. Nearly all were in the South, where obesity rates are particularly high.

"I think (the obesity theory) makes sense," said Michelle Martin, a breast cancer researcher at the University of Alabama at Birmingham. She was not involved in the study.

As for breast cancer deaths rates, the good news is they've been falling for both black and white women, most likely due to earlier diagnosis and better treatment.

But the white death rate has long been lower, and it's been falling for a longer time and at a steeper rate.

Over a decade, the white breast cancer death rate fell from about 25 to 21 per 100,000. The black death rate dropped from about 34 to 29 per 100,000.

In other words, there was a 38 percent difference between white and black death rate in 2003, and a 42 percent difference in 2012.

Again, researchers aren't exactly sure why, though they note that higher proportions of black women are diagnosed with late-stage breast cancer and with a particularly aggressive form of the disease.

Also, in the South at least, black women tend to have more additional health problems at the time they're diagnosed, Martin said.