Anyone know what the measurments on an ultrasound mean?!?!

Kennedy • Married to my soulmate since 7.4.2020!💍💞 We have a 4 year old son, who took a year and a half to conceive! We are expecting baby girl March 11th 2021 after 2 1/2 years of ttc with fertility treatments 💖
So I was looking at my 13 weeks (measuring 13+2) ultrasound and I've read they are supposed to be about 3 inches long... but when I calculated the centimeters into inches it says that measures 5.5 inches! I looked it up and that's a 18 week measurment! Is that the baby measurement or my uterus measurment?!? I'm freaking a little here