Hi K.,
Depending on where you live, there are several great organizations who would be happy to take the items (call first to make sure they accept them.) One that comes to mind is WINGS (I think it stands for Women In Need Growing Strong.) WINGS helps women in domestic voilence and homeless situations.
Another is in Palatine. The thrift-store is called Twice Blessed and it's on Northwest Highway (near Quest school.) The store is run by volunteers and supports Misericordia - an organization that helps residents with mental and physical disabilities that is located in Chicago.
I don't know about WINGS, but at Twice Blessed, if the items are not suitable enough for sale or if they don't take them, they will donate them to the Salvation Army so it just keeps getting passed down to organizations who may benefit from them.
Good luck with your sale. Of course if you wanted to try to get some extra cash out of the maternity clothes - maybe check into maternity resale. The name of the shop I'm thinking of escapes me at the moment, but it's in Schaumburg on Roselle road, just north of Golf, west side of the street next to Plato's closet...(Clothes Mentor, maybe? I think there are other locations in Naperville as well.)
Good luck.
Jen