| Primary interatrial foramen | |
|---|---|
 The developing human heart, at day 30. The ostium primum joins the primitive atrium and lies beneath the septum primum (top middle) and becomes progressively smaller as the septum grows in size. | |
| Details | |
| System | Cardiovascular system |
| Location | Developing heart |
| Identifiers | |
| Latin | foramen primum |
| TE | interatrial foramen_by_E5.11.1.5.2.1.1 E5.11.1.5.2.1.1 |
| Anatomical terminology | |
In the developing heart, the atria are initially open to each other, with the opening known as the primary interatrial foramen or ostium primum (or interatrial foramen primum). The foramen lies beneath the edge of septum primum and the endocardial cushions. It progressively decreases in size as the septum grows downwards, and disappears with the formation of the atrial septum.