Someone help

[Mumnet123]

Can someone please educate me on this.

So I had a period on the 11th November 2020 this lasted 5 days,
I found out I was pregnant New Year’s Day,
On the 14th January I had a ultrasound, by LMP I was meant to be 9weeks 3 days.
But it turned out I was 7weeks 3 days instead.

I had sex on the 1st of December.
But how I work it out from my ultrasound from gestational age, I would of conceived around 9th-10th December. And I didn’t have sex then.

I’m sure I had some implantation bleeding on the 12th of December as I thought it was very weird as it a weird ‘period’
But this is very weird due to my ultrasound date.

I was thinking did the ultrasound look at date I implanted. What’s been confusing me is how do they look at the measurements of a fetus as it doesn’t grow until it implants. And I’m sure I implanted around the 9th-10th.

Sperm can hang around for up to a week, ovulation can be late.

My last baby was conceived 2 days before my period was due. Regular as clockwork cycles. I knew exactly which "session" had caused it because it was the only time in ages.

Can't be more technical than that. Just thought I'd share my experience because it still feels mental now.

I seen on here that ultrasounds look at around the time embryo implantation!
And thankyou :)