At what stage can pregnancy be detected on a abdominal pelvic ultrasound?

Can pregnancy be missed very early on a abdominal ultrasound?

Can they tell that your uterus is changing... Ect.

I had a scan 3 weeks after I had sex and one 6-7 weeks after sex.

Do you think they would have seen something if I was pregnant. I'm VERY worried that I could be but it was just overlooked.