Yeah because in December I had a full normal period then in jan I only had a slight bleed and that was just the once when I went to the loo but that was it!it! I just didn't know whether I did the pregnancy tests too late so the hormone didn't show up? This month I tested the day I was due and it came up positive straight away.
I would still go off your december date and class the january 'spot' at implantation bleed... Often ladies get a bit of spotting when there period was due... With my first i had a 3 day bleed. For the sake of dating your pregnancy the MW is very likely to count from the first day in december.