You need to do the scan as soon after the bleeding has stopped which will give an accurate measure as to whether the Utrogestan is doing its job properly. I usually time it to go about 2 or 3 days after would not like to have it during the bleed if not necessary. And yes it does build up very quickly, I once went 2 weeks into my cycle and it was already at 7mm and just after my bleed when I went the following time, it was down to around 4mm.