First visit question

I have my first appointment January 13th, however i havent had a period since October 24th therefore I'd be about 10 week today and 11 weeks at my appointment (according to those online calculators that i beleve arent even close to accurate) . With that being said, my doctor informed me that no ultrasound will be done until 16 weeks...Uh how do they know 100% how far i am without an ultrasound? I dont even know if they will be checking for a fetal heartbeat.Literally so stressed because i just want that 100% assurance that my baby is okay and I want ro know for aure how far along I am, not just by assuming accordin to online calculators.