Measuring further then you thought.

I went to the ER for something else but they did a pelvic ultrasound. He said my little bean was measuring 5.5-6 weeks. Wrote 6 weeks on other paperwork. But if going by the FIRST day of my last period I thought I was 4.6 weeks…? I’m super confused. I track my period here, flo and premom. So I know I wasn’t off lol. I also bleed 9 days, I mean it was lighter then my usual flow but still pad worthy those 9 days. I actually took a preg test after it ended cause I thought my cycle was weird but thought why am I doing this? Haha. I’m interested if anyone else measured further then they thought. Could they have mistaken?! My aunt bleed the whole 9 months so I know it’s “possible” to have bleeding but like I don’t understand tbh.