Tony, from everything you posted before, she conceived in late September. That would not have been you. You could run into a problem if you did contact her and ask, she might begin to think you look like a useful guy to convince he is the dad. I wish you would stop obsessing over this, she is halfway through her pregnancy and never tried to contact you. She would have contacted you if she even thought you had a chance of being the dad, because she would have wanted to be sure it was her boyfriend's. Please don't put yourself into the position of being taken advantage of.
If you want to get back with this girl and have a relationship and stand in a father role to her baby, by all means call her and say you've heard she is broken up with her boyfriend and see if she wants to be with you, but otherwise, leave it alone.
Annie there is no way I would want to be with this girl. And if I was to contact her and she said there might be a chance or she thinks it's mine I would demand a DNA. I've worked the dates out and with me having intercourse a few times in August and early September and her having that scan in early December there is a chance that I might be the father, I've found out in the UK that your first scan is anywhere between 11 and14 weeks and that matches up exactly when I was sleeping with her, that's all. It's the last thing I want. I'm in a relationship myself, not when I had sex with this girl but since and I am very happy but I would hate to think in a years time there could be a child out there that could be mine. I am going to get my friend to try and find out the due date and if it matches up with any time I slept with her I will contact her and if she says not a chance it's her boyfriends I will leave it there as it is her prerogative but at least I've done the right thing.
From everything you have said, she conceived around September 26. But you are saying now that you don't really know what the date was on the ultrasound. Please, please stop using phrases like "that matches up exactly." You don't have any way to know if anything matches up exactly! You have a three-week timeframe if it's "anywhere between 11 and 14 weeks."
All I can say again is that you need to trust that a woman who is pregnant and suspects the baby is from someone other than her boyfriend will take steps to find out. Just read the posts on this community, the women are freaking out and really, really want to be sure. If she thought it was your child, she would have contacted you for a DNA test or at least to warn you that one would be upcoming once the baby was born.
My suggestion is that you calm yourself and stop saying it is exact or very close. From the info you've given so far, it is not exact and not particularly close.
Not that I don't appreciate that you are being responsible, but I think you are scaring yourself unnecessarily.
If you want to test the waters without looking too interested, you could have your friend post something from you on his social media page (or post something like "Saw Tony the other day, he's doing fine") and see if she contacts your friend for your contact info.