The number of American women who have died as a result of pregnancy or childbirth has doubled since 1987, making the U.S. one of only eight countries -- including Afghanistan and South Sudan -- where the rate of deaths among expectant moms is rising.

One key reason: Rising obesity rates, experts say.

In 1987, there were 7.2 deaths of mothers per 100,000 live births in the United States, according to the Centers for Disease Control and Prevention. In 2011, that number more than doubled, jumping to 17.8 deaths per 100,000 births, CNN reports.

While no single factor explains the increase, obesity-related complications such as hypertension and diabetes are a key contributor, experts say. Other factors may include the dramatic increase in the number of cesarean section births, a lack of access to affordable, quality healthcare and, more women giving birth at older ages.

Record-keeping changes might also explain some of the upward trend, experts say. It's now easier to identify pregnancy-related deaths because there's now a box for it on the standard U.S. death certificate used in most states.

"There were changes like this over time and this is why we believe that part of this increase is due to better methods of identification," said Dr. Andreea Creanga, a researcher for the Centers for Disease Control and Prevention's Division of Reproductive Health.