I would be inclined to go by scan dates if you have nothing else to go on kwim?
The pg tests these days are very sensitive, I did mine late in the afternoon/early evening when I was 3 days late & it was absolutely definite!

Having said that I am reluctant to base it only on scans as I have had experience of them being waaay off... with DD I had no idea when my LNMP was, we went with the date given by my 8 week scan & she was born 2 days after...brown wrinkled, peeling & clearly very very late.

So if you have anything else to go on, take that into account kwim? But I don't know if the test itself is much help in dating the PG.

Fortunately this time my 8 week scan matched my LNMP, plus I'm pretty sure of when I concieved, so no doubts this time