Should I be worried ?

Kiera • 21 , expecting my baby boy Noah October 17th , 2015 💙 he will be my first !
I'm 36 weeks and my doctor told me there is no need to do an ultrasound. Which I haven't had one since 20 weeks and my question is how will they know the baby's weight ? or am I just overthinking it ? Tomorrow I start my weekly visits .