He said they would do it when they have time it says nowhere that only 1 or all girls will have it if he is gonna give pregnant models an birthing scenes to all girls then i think eventualy he will make atleast some pregnant variant scenes for each so turning an old sex scene in a pregnant sex scene if drawing stuff from scratch takes to long
Sure, he never said that there's be only one scene or so per girl, but the planned content for the game is already way, way bigger than anyone would have ever imagined and spending even more time on pregnancy content won't be high up on his agenda. You may of course hope for it, but I personally wouldn't expect it, especially since there's an actual lack of stuff like blowjobs, boobjobs, anal, etc. and scenes in general, which will probably get a higher priority.
The problems with many old scenes is, that they weren't planned for pregnant content and thus can't really be adapted easily.
As said, that's just me saying it.