Early Gender Scans?

So, my OB offered me a 4D gender scan early at 14wks. Some people even get them at 13 there! She said it's accurate and if they can't tell they do a free re-scan in 1-2 weeks. Has anyone else had this done? Did the sex of your baby change as you got further along? I'm so afraid I'll go and they'll tell me something and when I go back at 20wks tell me they were wrong! I am very eager to shop but don't want to shop for the wrong gender! Help!