What Causes Delay In Periods?

hi
i was supposed to get my period on dec 28 2010 but havent got yet,,i checked if i am pregnant the result was negative,,
i have pelvic ultrasound scan on 24 jan 2011 is it safe to do that,,will it affect if i am pregnant..

also i was detected of polycystic ovaries 3 months back i have finished the tablets,,what should i do is this why i missed my period,,i have been trying baby from past 1 yr