6w5d but only measures 4 and a half weeks

Hannah

According my lmp, I am 6w5d but just been for a scan that is only measuring 4 and half weeks. They said I must be earlier than I thought but it’s been 2 and half weeks since my first bfp.

If a test is showing positive that long ago, how can I only be 4 1/2 weeks??

Advice please. I feel like baby must have stopped growing 2 weeks ago