To cite a web site, follow this basic pattern:
Author AA (if any). Title of page [Internet]. Place of publication (if any): publisher; date of publication [date last updated; date you're citing it]. Available from: URL
Understand your risk for congenital heart defects. Dallas (TX): American Heart Association; 2013 [modified 2013 Feb. 4; cited 2013 July 5]. Available from: http://www.heart.org/HEARTORG/Conditions
/CongenitalHeartDefects/
UnderstandYourRiskforCongenitalHeartDefects/Understand-Your-Risk-for-
Congenital-Heart-Defects_UCM_001219_Article.jsp
Not all of these elements may be available, but if you're unable to find more than a couple of them, you may want to reconsider whether this is a source you should be using for your research.