I was tracking my perios With glow Which told me I ovulated on September 8. My fiancée lives and works out of town. He spent the holiday weekend with me September 6 through the 8th to which we had sex.  In calculating how many weeks I should be on the 1st day of last period, i am 12 weeks 3 days.  Since the eight week scan they have told me I am measuring a week and a day ahead.  Again today at my sono they say that im a week 3 days ahead my fiancé gets the funniest look on his face hearing this.   How is this baby measuring that far ahead when I know when I conceived?  is it just going to be a big baby?