How To Confirm The Correct Menstrual Cycle Dates?

I had a regular menstrual march 22, 2011. lasted for 7 days and thats pretty normal for me. then i had light spotting for 4 days april 22. I did take a pregnancy test it was positive confirmed by doctor. BUT when asked first day of last menstrual would it be march ? or april ? thats a big difference from being 6 weeks and 10 weeks pregnant ?.