I'm with the others...why the heck not?
However saying that, when I was just preg with the Little Guy, a friend of mine had a shower organised for her by some of the girls from work. I overheard a few of the people invited (who went and bought gifts etc) saying that it was a bit rude for a second baby.
Who cares though?
It's supposed to be fun. Call it a celebration of your pregnancy or something.