@andrewmathias8 Hmm. Okay, just for due diligence (and to make sure that I’m not entirely off base here):
Osmosis (https://www.osmosis.org/answers/interatrial-septum):
- “The interatrial septum forms during the first weeks of fetal development…”
Wiki (Interatrial septum - Wikipedia):
- “The interatrial septum forms during the first and second months of fetal development.”
I believe, if I’m on the right track, that there is some nuance that’s being lost in the card and might could represent an opportunity for some additional clarity.
As I (currently) have it:
- The interatrial septum itself forms during fetal development.
- The primum and segundum do indeed fuse after birth, though this leads to completion/closure of the atrial septum rather than its formation.
This might admittedly be a relatively minor point, but I wonder if the card couldn’t benefit from some clarity to refine some of that nuance (FWIW this card was brought up in a Reddit post as being the source of some confusion for a number of users, which is what led to my initially digging into this and hoping to find some refinement for the card).
Perhaps something akin to:
- During fetal development, the septum secundum and septum primum fuse to close the {{c1::atrial septum}}
(‘close’ rather than ‘form’ seems like it might be more appropriate here?).
Possible I’m overlooking something, of course, but wanted to follow up out of earnestness. Let me know what you think. 
@Brian_BH Ok, yes, I see the distinction that you’re trying to make now! Could you please submit this suggestion again with those additional details and I’ll bring it up with the maintainers?
On it.