My 12w or 13w Ultrasound

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Had an ultrasound today which made me feel so happy to see my baby, heartbeat was fine and my little munchkin was stretching. I thought I was around 12w, the first day of my last period was September 15th. But the doctor told me I am 13w. I wanted to ask the doctor what the gender was so bad lol can anyone quess the gender using the nub theory