Need help ! Who is father based on ultrasound dating

[Ashlyy88]

I had sex Jan thirteenth I believe I had a period the eighteenth also he pulled out …. had sex other times with one other person after that and continued to do so ….. I got a light positive test on February sixteenth , February twenty first I got my hcg level took it was 113…. Ultrasound done March 18th showed a due date of October 31st.. 7 week 4 days ….. when would my
conception be ?

Conception around 8th feb I'd say more likely the second one,

To work out approximate conception date you count back 38 weeks from your due date.

The second one.

This. I had a baby due on October 31st and she was conceived on her Dads birthday… however, cycle lengths do vary and this was based on the average (changed by 3 days after ultra sound as I have a short cycle)