When did I get pregnant? It is so confusing

[Mum1and1]

So over Christmas I found out I was pregnant and I cant understand when did it happen.
Here it goes.
In the middle of the week sometimes around 20.12 I found out from pregnancy test I was pregnant. Last sex I have had was few days before that, around 15-16.12.
On 15 January I went for private scan and from measurements they have calculated I was 7weeks 1day so due date was set on 31.8.24.
I forgot to ask them at the time of the scan.
Is there any change I could get pregnant from that sex on 15-16 December? Or it must have been before? I was one 1-2 days before menstruation should start.
I am only confused as because if I do use those online conception calculators, they all put conception day way earlier, on 9.12. and sex leading to conception even earlier. Based on ultrasound calculation from January or due date times calculations.
I didnt do calculations based on last menstruation because I have had miscarriage back in Jun 23, and since then it was a bit off. Could I get pregnant 1-2 days before menstruation should start? Unfortunately week after scan I have had another miscarriage so any further scans are not possible.

Hey, I think implantation usually occurs 6-10 days after conception. I think pregnancy test can show positive after implantation? Are you trying to calculate your due date or is it just curiosity?

It's more of the curiosity. Due date they have told me on the scan which was 31.8

Well if you were 7 weeks on the 15th January just count it back, they count it from the first day of your last period even though you weren't actually pregnant until 2 weeks later so 7 weeks back means 27th of November For fist day of period) and let's assume you ovulated somewhere in and around 11th December that's when you conceived.