9 weeks measuring 8...worried

Lindsay • Baby #4 on the way!
I just had an ultrasound and baby is measuring a full week behind. Strong heartbeat seen, dr doesn't seem concerned. Should I be? I charted, bbt, and opk, so I'm pretty confident about my date. He did the ultrasound abdominally today, not sure if that affect accuracy or not. Anyone experience this and go on to have a healthy baby?