It's been six weeks since Dec 24th
I’m just not extremely positive on the 24th being the day. I had an IUD removed not too long ago, so my periods were pretty irregular and I’m not great at keeping track. But thank you I wasn’t sure which way it was being calculated!
Call it the 24th. It is close enough. Typically you ovulate 2 weeks after the start of your period. So 2 weeks before conception is roughly when you would have had your period. I wouldn't worry about getting it exact. The doctors can measure the baby by ultra sound and assign you a due date off of that if you are more comfortable that way.
6 weeks is right, it's calculated by first day of last period not date of conception :)