The number of newborns in Europe affected by heart defects appears to have fallen in recent years, but it’s not clear why, says a new study.

A team of European Union researchers analyzing millions of birth records found that the number of European babies born with heart defects fell from around seven in every thousand births in 2004 to around six per thousand by 2007- a drop of around four percent each year.

While any drop in the numbers of babies born with birth defects is good news, experts noted that the decline is a modest one and the researchers can only speculate about the reasons for it.

There is cause “for a high level of skepticism,” said Dr. Joe Simpson at the New York-based March of Dimes foundation, a non-profit organization that works to improve babies’ health.

“It would be lovely if… true,” Simpson told Reuters Health, but given the lack of explanation for the change, “whether it persists over time remains unclear.”

For the new study, a team led by Dr. Babak Khoshnood at the National Institute for Health and Medical Research (INSERM), in Paris, France, looked at data on congenital heart defects collected in 16 mostly western-European countries between 1990 and 2007.

They found 47,000 cases of congenital heart disease among more than seven million births.

According to the report, published in the Journal of Pediatrics, cases of the most severe forms of CHD - including conditions like Hypoplastic left heart syndrome, when the left side of the heart doesn’t develop properly - has held steady at around five per 10,000 births since 1990.

But less severe conditions, like ‘hole in the heart’ syndrome, decreased from around 50 to 40 cases per 10 000 births, between 2004 and 2007. For a country like France, with around 780,000 births a year, that means about 780 fewer babies born with heart problems every year.

Khoshnood speculates that increased folic acid intake by European women is a possible cause of the decline in defects. But, “we don’t have the data to know for sure,” he added.

A recent study in Quebec, Canada, found that the number of babies born with heart defects dropped after 1998, shortly after the government began adding folic acid to staple foods like cereal and bread.

Folic acid fortification programs were introduced in both Canada and the U.S. in 1998 to reduce the number of serious birth defects in the spine and brain, known as neural tube defects. But there are