Gestational Age Vs Uts Age

Im confused po kasi paano mgcount ng pregnancy ko. Di po kasi ako regular, i had my mens last Jan 20 so nung ngpaultraaound ako last week sabi ni dok 8 weeks. Pero according sa uts, 5 weeks pa lang. Nalilito ako kung paano ko ba susundan ung development ni baby, kung normal ba ang development niya for the age or not.