I either say "pop-pop" which is what DD has christened this baby, or i say "he", mainly because DD is determined it's a girl and i want her to be somewhat aware/prepared that it might be a boy. I use "it" sometimes too, which doesn't bother me at all.
first one was stinker, second was booger and this one is squirt. I have no idea how these nicknames came about, I blame DH because he refuses to name them ahead of time so we have to call them SOMETHING
I almost always said 'he' 'cause I just knew it would be a boy. We also had little nicknames, one we used most often was 'KB' (which is the first letter of the girl's & boy's name we had originally chosen, although we didn't end up using either!) I hated "it" because he wasn't an "it" -- he was my baby!!! So he, KB, the baby, whatever... that's what we went with
the first time, I alternated between 'he' and 'she' just to cover the bases
the 2nd time we found out, but before we did, we used 'he' 'cause it was easier
this time I'll probably use 'he' also, 'cause I have 2 boys and am assuming it's another one - we aren't finding out, though, I don't think.
We use a pseudonym as well. Newbie or Moon or Bean. But also "the baby" and somehow we started calling it "the newbian". Mostly dd calls it "little brother OR sister" when she's talking to the belly, which is pretty cute.