According to iui i should be 6.5 weeks along but vaginal ultrasound measured sac at 5 w 1 d....confused?

Repeat US in 1 week: Ultrasound assessment at this early stage is accurate in defining several important things, including fetal pole, fetal cardiac activity, heart rate, the presents of a yolk sac, and the shape of the gestational sac that contains the fetus. It can be somewhat inaccurate for the gestational age. A follow up ultrasound in 1-2 weeks and checking blood b-hcg levels would help.