well according to this hcg levels are normally around 300 at 4 wks...
http://www.babymed.com/hcg-level-in-early-pregnancy
in the UK we don't get our hcg levels checked. I'm 8 wks pregnant with my 3rd child & have never had my hcg levels checked. think it just gets women all worked up for nothing.
280 is pretty low.. it can mean ur in the really early stages. i remember i was in the 300s when i was 3 weeks. and u cant see anything in an ultra sound until ur 5 weeks
I would say you're around 4 weeks pregnant with a reading of 280, which would be why nothing is showing on an ultrasound
yet. your baby is only the size of a mustard seed so you may only see the sac that early. most ultrasounds are carried out around 12 weeks (UK)
:)