BFP but line isnt getting any darker

Hi all. I’m new here and this is my first <a href="https://glowing.com/glow-fertility-program">IVF</a> cycle

FET 23/12

Got a BFP 30/12. Have tested daily since but the line doesn’t seem to be getting any darker. Waiting for blood test in 2 days. Is this normal ? TIA