The ostium secundum (or foramen secundum) is a foramen in the septum primum.
It should not be confused with the foramen ovale, which is a foramen in the septum secundum.
Read more about Foramen Secundum: Discovery, Embryological Development, Anatomical Position of The Foramen Secundum, Closing During Birth, Clinical Significance, Medical and Surgical Care of Ostium Secundum Atrial Septal Defects, Prognosis