Severe complications from childbirth, including heart attacks and strokes, have been rising in the United States, although they still remain rare overall, according to a U.S. government study.

In 2008-2009, there were 129 cases of severe complications - including heart attack, stroke, severe bleeding and kidney failure during or after childbirth - for every 10,000 women who delivered in a hospital, said researchers at the Centers for Disease Control and Prevention. (CDC)

That was up 75 percent from a decade earlier.

At the same time, complications during women's post-delivery hospital stay also rose to 29 cases for every 10,000 women, up 114 percent from 10 years before.

Serious complications and deaths from childbirth still remain uncommon in the United States. Over four million women give birth each year, and this study, which appeared in Obstetrics & Gynecology, found about 590,000 cases of severe complications over 11 years.

"We don't want to send the message that pregnant women should be afraid," said William Callaghan of the CDC, who led the study.

But he added that it's well-documented from other research that more women are giving birth at older ages, are obese or have certain health conditions, such as high blood pressure and diabetes.

There are also more young women with serious conditions, such as congenital heart conditions, who are surviving and having children.

"The characteristics of the pregnant population are changing," Callaghan said, noting that it's thus not unexpected that rates of certain complications might rise.

Another recent CDC study found minority women at particular risk. Between 1993 and 2006, minority women accounted for 41 percent of all births nationwide, but 62 percent of all pregnancy-related deaths.

Black women were at greatest risk. For every 100,000 babies born to African-Americans, 32 to 35 mothers died. That was roughly four times the rate among white mothers.

Heart problems were the most common cause of death, and Callaghan's team found that one childbirth complication - the need for cardiac surgery during or after delivery - showed a "dramatic" rise over time.

But it was still rare. In 2008-2009, just under 5 per 10,000 women needed a heart procedure during delivery - though that was up 75 percent from a decade before.

Callaghan said the bottom line for is to be as healthy as possible before pregnancy, such as losing weight if you're obese and getting high blood pressure and diabetes under control.

"Not all complications can be avoided, of course. But the best outcomes happen when a woman is as healthy as possible going into pregnancy, he added.