Not accurately with an ultrasound. Up until 16 weeks, genitalia looks identical for both sexes. There is a blood test that can be done, but it's very expensive.
There are some theories regarding sex prediction via u/s at that stage, but the accuracy is less than 50 for these methods. Sorry, you'll have to wait like the rest of us unless you're getting an amnio.
Re: 13Wkfirst ultrasound
BFP #1 - 01/12/12, EDD 09/12/12, Medical Induction @ 21 weeks 05/03/12
BFP #2 - 10/30/12, EDD 07/04/13, Natural m/c @ 5 weeks 11/01/12
BFP #3 - 02/07/13, EDD 10/12/13
Dx: Incompotent Cervix