Welcome to HCM.
Basically ovulation occurs 14 days before expected date of regular period. But it may differ with irregular period.
GA is calculated from 1st day of LMP (in your case 17th Sept) to till date of wk.
USG scan not able to give detailed fetal profile in early wk. Gestational sac
and fetal pole
are visible on TAS on 6wk and 8 wk respectively and on TVS 4wk & 6 wk respectively.
So consult with your gynecologist and take advice to smooth Antenatal Period.