6 weeks in count but 5 weeks and 1 day sa ultrasound

hello mga moms.. ask lng po kc 6 weeks na aq pag bilang ning last mens. ko pero nung nag pa ultrasound aq 5weeks and 1 day last jan 11.. is that normal? pinapabalik aq 2-3 weeks para pa ultrasoind daw ulet.. mejo praning mom here.