How accurate is an ultrasound, Based on first day of last period I am 7 weeks, ultrasound says 5, why is this?

How accurate is an ultrasound, Based on first day of last period I am 7 weeks, ultrasound says 5, why is this?
Question by natasha_526_tx:
I am pregnant with my second child. My first pregnancy and delivery went awful. At the time of my last ultrasound the ultrasound dr measured my daughter to only be the size of a 36 week old which put my due date from February 25th to March 15th. Following that ultrasound I started having severe hypertension and only 2 weeks later started showing

ultrasound physiotherapy