We have been ttc # 2 for over 2 years now. We got pregnant with our DS right away and got pregnant right away when we first started trying for # 2 (but had a m/c).
We did testing with my OB after about a year of trying...and then when we realized an OB wasn't really going to be of much help, we started going to an RE. They did more testing and nothing has popped up for us either. They are calling it "unexplained infertility".
Currently we are doing Letrozole and Ovidrel and will be doing an IUI with the RE. (We've done 5 IUIs already...we did get pregnant that way once, but lost that baby as well).