Help pregnancy estimation!!!!

Has anyone ever gotten pregnant and went by their glow app estimation but when they went to the doctor's they we measuring smaller? Idk if im making sense, ok so according to the glow app im 6 weeks and i went to the doctor because i spotted for one day and freaked out because ive had 5 miscarriages already. The doctor told me he sees a gestational sac but that i look about 4-5 weeks. I have PCOS and my cycle are long sometimes could it be i ovulated later then i thought and that's why im smaller?