When you were growing up, did your mother call you or refer to you as her “baby” only until your next sibling was born? ~

Nope.  There were no siblings after me.  I literally broke the "mold".

I was only sixteen months old, so I don't remember. I can't ask her she passed away 5 years ago.

I don't think so. I was too young to remember.

I'm an only child, so I've always been the baby. My mom affirms that by saying things like "you'll always be my baby". :P

I don't think my Mom ever referred to me as her "baby" .. even before my younger sister was born.  She was more about ... GROW UP, please.  
I just have one son.  I can't imagine him being thrilled with me calling him my "baby". 
I should have done that when he was in the Navy in his Officer's uniform, however.  My right as his Momma!  

Mum was still grandad's wee girl long after we were born. 

I don't think so.

I remember that monster's birth.  He wasn't a monster back then.  He was a baby.  But, knowing what I do now?  I would have put a pillow over his moron head.  

No she did not.  I never really got to be "the baby" for any noticeable period of time.  I was the first born.  The oldest.  Then rapidly came my sister.  Then came another sister.  Then a brother.  Then another brother.  I was their fearless leader.  My siblings called me "Big Boy".