So yesterday we were able to hear the heartbeat for the first time. It took the doctor a couple seconds to find it (which freaked my wife out a bit), but it's there, it's strong, and it's at 160 bpm. It was neat, but I've yet to have a feeling close to the feeling I had when we were told it was twins (if you just tuned in, it's not twins).
I'm going to a moment to note, but not say I agree with, that most, but not everything, we've read says that over 140 bpm means it's a girl. I don't hold any stock in this whatsoever (I won't rejoice in it being a girl until we have conclusive proof). There are a lot of places online to take a survey and find out what sex the baby is. They are all bunk, IMO.
So anyway, to recap: no twins, strong heartbeat, wives' tales point to girl.
On a side note our friends Sarah and Eric just had their 4th girl. And in case we do have a boy I've arranged a deal with Eric to trade him a girl for our boy. Very rumpelstiltskin I know, but I'm frightened to have a boy.