I'm sure posting in Fanfic Related would get you a slew of titles, Snarla.
There have been many stories over the years (which can be found on the Archive) which have dealt with Clark going away (to NK, or losing time somehow, or L&C meeting in their younger days and being separated by circumstances) and returning to find Lois has a child already.
Off the top of my head, I can think of Sheila Harper's fabulous "Faster Than A Speeding Bullet" (there was an original version set I believe in S3, then a post-marriage reworking of it for TUFS) and ML Thompson's "Without Checking the Water Level". The first NK story like that which comes to mind is Piper's "Three Grandmothers" (though that's a bit different, because Clark doesn't come back).
I know I've read many others, though. Start a thread in FF Related, and I'm sure others' memories will be better than mine this morning.
Kathy