Pregnancy calculation

I'm trying to calculate how many weeks pregnant I would be my last regular period was December 30th their irregular and then the next I was spotting old blood(assume implantation) it happened for a day or 2 January 31st and ever since then no period my doctor told me to take a pregnancy test at the end of February please give me calculations on how many weeks I'd be pregnant or does it sound right to take a pregnancy test at the end of February just please give ur opinions wonderful mommmies!!!