Ultrasound day

Had my 20 week ultrasound today. Everything went pretty well and found out we were clearly having a boy. Not what I was wanting but I'm still happy. Afterward I had a doctor's appointment and she told me I had to have another scan in your weeks because the babies position made it hard to see the heart and parts of the spine clearly. I was told this was very common and told me if this was her ultrasound then she would not worry, but of coarse it still makes you wonder. Had this ever happend to anyone else?