Thanks everyone... Interesting theory @Blossom74 but unfortunately I don't think that's it... I have stark white BFNs at other times and only five times in the last eighteen months I've got the squinters for a couple of days before fading out. I test every cycle (addict!) so most cycles don't show any hCG at all and they are obvious BFNs. I think it is more likely implantation failure as I didn't have any trouble falling pregnant prior to starting meds in Nov 14, and have only had chems after that, so something is going wrong. The timing all works perfectly for a chem unfortunately I will have the hysteroscopy early April and see what shows up from that - I'll let my FS know about the chems, the line is too significant to be anything else.