I’m in very early pregnancy but I ovulated late in my cycle and am measuring behind according to LMP.

My doctor ordered a sonogram due to previous MC and they didn’t see anything. It was a transvaginal US. I was asked to go back after a week and, when I did, they found a gestational sac but nothing else. That ultrasound was done externally. Is it normal to have an external instead of vaginal US that early? Could they have missed the yolk sac because it was external?