had my cycle sept 13th 2014 and its a 31 day cycle. I took a pregnancy test on the 11th of oct and it was postive. I got a ultrasound today (vaginal)and the fetus only measured 5 weeks and 2 days, now how it that possible im suppose to be 6 weeks and 4 d

Dates in pregnancy. When the age of a fetus or a delivery date is calculated from the dates of a women's menstrual cycle we say "plus or minus 2 weeks". Menstrual cycles vary in length and regularity and it is impossible to know the exact moment the pregnancy started. During an ultrasound multiple different measurements of the fetus are taken and while this may be more accurate it is also not exact.