Well done ladies! I saw the little doodad on BW's chart when I was having my morning stalk - now I'm most vexed because I was predicting BW's BFP for today.

hCG is from implantation, not fertilisation. It's made by the placenta to start to comandeer your body into making loads of new hormones as the placenta can't do that at the beginning; it needs to grow before it can take over.