Figures released Tuesday at a medical congress in Paris revealed pregnant women with heart disease face a 100-fold increased risk of death, with the danger for offspring multiplied by ten.
Analysing data on 1,300 women gathered since 2008 from 28 countries across Europe, researchers reported 13 maternal deaths -- one percent of the cohort -- among expectant women with pre-existing heart conditions.
In healthy women, the average rate of maternal mortality in Europe is about one in 10,000.
Of the 1,300 women, 869 had congenital heart disease, 333 were vascular heart patients, 79 had cardiomyopathy and 24 suffered from ischaemic heart disease.
Cardiomyopathy causes the heart muscle to become thicker and more rigid than normal, while ischaemic heart disease is characterized by reduced blood supply to the heart.
There were 59 cases of fetal deaths, or 4.5 percent of the total, about ten times the normal rate of mortality for European nations.
Data on the relationship between heart disease and maternal mortality is scarce, and randomised clinical trials are not possible, noted Jolien Roos-Hesselink, chairman of a European Society of Cardiology (ESC) data registry set up in 2006.
"The only way to improve our knowledge of the factors which determine outcome in pregnant women with heart disease is to gather data on a large number of pregnancies and try to find patterns of outcome which correlate with management strategies," he said in a statement.
Roos-Hesselink said the figures, reported at the ESC's annual congress, which ends on Wednesday, was an interim analysis, and that more data was needed for a better understanding of which types of heart disease pose the highest risk and what treatment may be most effective.