Already booked in for a 35 week scan??

Hannah • Trying for baby number 2
Hi ladies I'm 17 weeks pregnant I had my 12 week scan at the begging of July had my results back as everything fine and low risk, booked my 20 week scan whilst there for end of August. 
​About a week ago I had a letter asking me to attend another scan for "growth assessment" booked for
​End of nov which would make me 35 weeks pregnant, I'm just a bit concerned as I don't know
​Anyone with a 3rd scan and especially booked in before I even had my 20 week scan!! :-S 
​Should I be worried??