Baby Size vs. Due Date

Hello Mamas!! So based on my last period, (July 26th-29th), and my missed period, (supposed to have been August 19th), I was counting that I was 18weeks and a few days along in my pregnancy. Finally had my first ultrasound today and the baby is only measuring 16 weeks and 2 days, so I was a little surprised. My question is, am I really 16 weeks along or is my baby just small? My due date has been changed twice now by two different doctors and I just want an accurate timeline. TIA!!!🎉