Researchers say while it may not seem like a large decline, treatment of heart defects is complex and carries a high risk of death, so even a small reduction is important.
This was the first large study to look at folic acid and heart defects.
Researchers say there could be other factors that led to the drop, but it still shows a strong connection between folic acid and serious heart defects.
Congenital heart disease is the most common type of birth defect. Previous research has shown folic acid reduces the risk of spina bifida and related birth defects.