Follower NPCs are a bit buggy and several have been known to vanish including Uthgerd. Bethesda disclosed in its patch notes for the current 1.6 beta patch that they can vanish if you pay off a bounty while they are in your service. There may be other reasons they can vanish as well that we don't know about.
The other thing is that when you get married, your spouse literally walks from Riften to your home, which can take a while. They can get stuck in physics somewhere along the way and be standing or walking in place somewhere between the two places. They can also get killed on the way to your home which is why it's not a bad idea to escort your spouse to your home.
If you're on a PC you can use the console command player.moveto 00091918 to see where she is. If you get a message back indicating a problem with the reference ID number it means she's vanished from the game. Otherwise, dead or alive, the command will teleport you to her location. If you aren't playing on a PC, check the Hall of the Dead and if her remains are not there then you can try traveling along the roads between Whiterun and Riften to see if she is stuck somewhere.
