Welp, guess it's time for me to start messing around with my save file. Since you don't know the number, can you at least tell me the name/description of the event chain that the game fails to trigger? Right now I'm at the very start of Act II.
I already modified my game files to make the event work so I really don't remember what needs to be changed to make it work properly, what I'm gonna suggest
should work to bruteforce the event but keep backup saves just in case.
Make sure you don't have any other pregnancies going on at the moment
Make sure you've already had the first sex scene with the dragon
Toggle switches #3561, #3575, and #4877 ON
Go to your bedroom and progress the pregnancy like any other
If the above doesn't work, you can try this instead if you just want the preg sex scene:
Make a save just outside the dragon's cave
Toggle switch #4878 ON
Enter the dragon's cave
If neither of those work then sorry but I can't help you, I won't be able to remember what I did to patch it in my game.