Cardiac arrest during childbirth is rare, but it may still be seriously under-reported, according to a new study in the journal Anesthesiology.

Maternal cardiac arrest is when, during or after childbirth, the mother's heart stops beating. It can be caused by excessive bleeding, heart failure, heart attack, preeclampsia, blood infection and amniotic fluidembolism - all of which can result in irregular heart rhythms or reduce the flow of blood and oxygen to the heart.

As well as potentially being fatal for the mother, maternal cardiac arrest can result in the infant being stillborn, although doctors will usually attempt a C-section to ensure the baby's survival. Studies suggest that the infant should be delivered within 5 minutes of maternal cardiac arrest to avoid any neurological problems.

The researchers behind the new study looked at data on more than 56 million hospital births. They found that, among these births, 4,843 cardiac arrests occurred.

This means that 1 in every 11,749 births resulted in maternal cardiac arrest - a figure that is twice what has previously been reported in medical literature.

The researchers also noticed that survival of maternal cardiac arrest improved from 52% in 1998 to 60% by 2011. Cardiopulmonary resuscitation often seemed to be successful, and the team notes that the survival rate of 60% is also higher than has previously been reported.

'Maternal mortality in the US remains unacceptably high'

"A 60% rate of survival from cardiac arrest is good, but maternal mortality in the United States remains unacceptably high," says study author Dr. Jill M. Mhyre, associate professor of anesthesiology at the University of Arkansas for Medical Sciences in Little Rock.

"This information will assist health care providers to deliver the most effective maternal cardiopulmonary resuscitation when both the mother's and baby's lives are on the line," she adds.

Dr. Mhyre's team found that the main cause of cardiac arrest was excessive bleeding, which accounted for 47.7% of all cases. Heart failure and amniotic fluid embolism each accounted for 13.3% of cardiac arrest events, and blood infection caused 11.2% of maternal cardiac arrests.

The study also found that older women, black women or mothers receiving care funded by Medicaid were most at risk of suffering maternal cardiac arrest.

A 2012 study in the Journal of Perinatal and Neonatal Nursingreported similar findings, suggesting that black women are 3.7 times more likely to die from pregnancy complications than white women. That study also hypothesized that maternal cardiac events are under-reported in the US due to inconsistent state reporting systems.

"These are rare high-stakes events on obstetric units, and team preparation is critical to ensure that everyone is ready to act quickly and effectively," says Dr. Mhyre. "Fortunately, physician anesthesiologists are experts in leading resuscitation teams for maternal cardiac arrest and other emergencies that happen on the labor floor."