Help!

I feel kind of stupid for asking this, but it's bugging me. I went in for my ultrasound on September 8th and the baby measured to be 12 weeks and 0 days. The lady measured the baby's head and said the head is measured at 13 weeks. Is that possible? Or does it just mean my baby is going to have a big head? Lol. Also, this app is saying I'm exactly 13 weeks today when, according to the ultrasound, I'll be 12 weeks and 2 days today... 😒