This sounds like one of those strange maxis mysteries, lol
Well this whole this was driving me a little crazy so I did a google search and found a chat on the Official Site. (It's like, I know I sent that pregnant sim to the lot I don't think I was dreaming, Sometimes I do dream about sims, I know I need a life. lol)
Chat
1-19-06
MaxisHunter!
"Pregnant women can now go to the community lots for almost their entire pregnancy."
So I guess lemonfresh is right about how far along a sim is in their pregnancy.