Confused by weeks pregnant 🤔

I went for my first scan and the baby measured at 7weeks 6days. I was expecting to have conceived only 6 weeks ago however as I came off the cerazette pill around 10th November and had some period like bleeding on 18th, unsure if this was implantation or withdrawal from stopping the pill as it only lasted 1 day. I haven't had an actual period for months just bleeding due to contraception. On ultrasounds do they measure from conception or add 2 weeks? I thought I conceived around the 19th as I took a clear blue test around 10th December which said 2-3 weeks. Any help would be appreciated, thanks!! ❤️