it really depends on the individual doctor/nurse/technician, if s/he is willing to tell you his/her best guess.
i read from many websites that it's possible n more predictable to tell fetal gender after week 15 but it's not as easy as week 20 or later. it also depends on the position of the fetus. u can ask but no guarantee.
what happened to me's that my OB told me one gender at one time (wk 11) and another gender at another time (wk 15). now my hubby n i agreed not to think too much.