Is there any reason he would think it is not a viable pregnancy, have you have a blood test that might indicate such? If it is just a lack of symptoms that prompted the Dr to write that, then i don't think you should worry too much.
Medical terms can often be alarming. Early on in a pregnancy i dont think it is unusual for Dr to write something like that on a referral for a dating scan. Infact i think my dating scan said that at 7weeks.

Try not to worry. (Don't ya hate it when people say that.) Good luck.