No ultrasound in third trimester

Will anybody else doctor not give them an ultrasound in the 3rd trimester ? I had my first one at about 10 weeks and then my second one (the anatomy scan) at 20 weeks and now I'm 35 weeks and she said I don't need another one because there was nothing abnormal about my anatomy scan , which I'm happy my baby is perfect but I thought I would get one last ultrasound in the 3rd trimester to atleast see if he's breech or measure how big he is but she said no :( I don't understand how she will be able to see if he breech or not if she can't even see him 
(I'm a first Time mom by the way so i might just be going crazy )