A doctor can give you an estimate on when you conceived. Because you have irregular periods, they might date you at date of conception instead of your last period like they normally do.
You'll beed to speak to your doctor about what they estimate your conception date to be - but these are never 100% and can be off a few days either side.
Juneflower, was the 12th-week ultrasound the first one you had? Since by that point, there is a margin for error of plus or minus a week, if you use it to date the pregnancy. That said, you would have conceived somewhere around January 25, plus or minus 7 days.
Yes it was the first one they did transvaginal and a standard one same day
What date was the sexual event you're worried about?
I had sex on January 12 and 25
I had sex on January 12 and 25
It pretty much sounds like the guy on the 25th would almost have to be the dad, since it would take a two-week margin for error to make the guy on the 12th the dad, and you aren't far enough along in pregnancy for the margin to be that big.