If your only 5 weeks then maybe the bean is still too small to be picked up on? If your HcG levels are still rising, then thahts a good sign.
check out this link - i was browsing it earlier, and i found it very informative. xx
http://www.baby2see.com/development/ultrasound_sonogram/first_trimester_scans.html#week5
Hope all turns out well hun, xxxx