How soon

Does anyone know how soon a doctor can see signs of a developing pregnancy on a transvaginal ultrasound? I ovulated April 5th and had one done 4/13 just to see if my uterus was fine. Everything was normal! But I noticed my doctor deemed me “pregnant” on my discharge papers.. but he didn’t mention it to me. Today I took a pregnancy test to confirm & it was positive! Is it weird for my doctor to not disclose that to me? Or do you think he chose to wait until definite signs of pregnancy was shown?