Help me figure this out!

This a little of a long story but here we go: I know when I had the first day of my last period (29/09, usually 4 day cycle) and we had sex on the 6/10 (only time we had sex). Taking into account I have irregular periods between 34-36 days. I had implantation bleeding on 25/10 which means I most likely ovulated (as I normally do) day 16/17 of my cycle. I know my body really well from 4 years of ttc. Sperm can also live up to 7 days before fertilisation (there is a variation here too) occurs which could easily of happened here hence I’m having a girl. On my hospital notes they have my last period as 24/09 based 32 days cycles. In my 12 week scan they jumped my DD from 11/07 to 30/06. In my 20 weeks scan yesterday they said baby girl is in the 3rd percentile of growth which means she’s 2/3 weeks behind what they are saying she should be (anatomy is healthy and normal) and now I’ve been referred to FMU for further testing which I have booked for tomorrow. Downs testing etc at 12 weeks came back as low risk. Should I tell them that I know exactly when I had sex and that they have my last period dates wrong? I’m having a planned c -section and I’m terrified that they will take baby out earlier than they think she will be. I’ve read that ultrasounds can often miscalculate EDD and due dates are often estimates even via scans. If I go by the scan dating, I would have conceived within 7 days of having my last period which is impossible