Are abdominal ultrasounds reliable?

Based on lmp I’m 6+4 but sac measures 4+5 and no yolk sac present. This was abdominal, is it possible transvaginal could have shown more? Doctor was certain transvaginal wasn’t necessary and told me to come back in two weeks. I don’t see how sac could be measuring early when I had my hcg checked a few times 2wks ago and they were good numbers.