How to tell when conception was

My ex girlfriend is pregnant and today on 12/14 she had her first ultrasound and the baby 7wks 3days old, and we are not sure who the father is, how what would the date of conception be if this is the case

[hoping43]

Approx 5weeks and a couple days ago.

[crushed266]

the last day of her period was on the 26th but we are not sure when it started

[lisas9]

Conception would have been 5 weeks ago give or take a weekish but anyone she has had sex with from 7weeks 3 days ago until she found out is a potential father and the only way to tell would be a dna test if there was more than one.

[lisas9]

The date from the scan suggests her last period started 7 weeks and 3 days ago as pregnancy is counted from the first day of your last period.

[Blue_eyes2012]

When's her due date so I can type it into a concepTion cAlender

[LynnBrooke]

Due date?

[acline93]

I think the best option is to keep an open mind that you possibly are the father, and then when the baby arrives, get a paternity test done. You wouldn't want to make the mistake of asking people online if you could be the father, and then find out that you aren't after you've signed the birth certificate. If I was a man, I'd definitely get a DNA test, but still be supportive because pregnancy is difficult for all women.