I went with my girlfriend for a sonar on 29/08/2013... according to the results, the baby is 9weeks 2days. This means (if I am correct) that the baby was conceived round about 25/06/2013. At that time we had broken up and she was supposedly sleeping with another guy. The last time we had sex was 15/06/2013 which is ten days before the supposed conception date. Realistically, is it possible that I am the father?

If there are issues surrounding paternity then a DNA test after the birth is the only surefire way of establishing the facts.
Gestational Dating is all based on Best guess.

Yes its possible. Conception estimates are just that, estimates. They can go 2 weeks either way. The only way you will know for sure is with a paternity test once the child is born. And you should have one before you sign an acknowledgement of paternity.

I would want to avoid putting everyone through that process (DNA tests)... if the dates were out by 1 or 2 days it wouldn't be a problem, but 10 days? Isn't that a bit unrealistic? The problem is we're not getting along at the moment and the thought of raising a child with her is scary at this stage... if the child's not mine it's going to cause even more pain, would rather just get things sorted out now... oh well, thanks for the feedback!

The pain is already done. A DNA test would relieve the pain of not knowing. Do the test for the sake of the kid. He has a right to know who his daddy is.

The pain is already done. A DNA test would relieve the pain of not knowing. Do the test for the sake of the kid. He has a right to know who his daddy is.

Good point!

Its unfortunate, but there is no way to be sure. Yes 10 days seems like a lot of time, but if you had intercourse on the 15th, it could take a few days for the sperm to travel and impregnate an egg. And calculating conception is an inexact science. So, as I said, it can go 2 weeks either way, sometimes more. There are only 2 100% effective ways to determine paternity. The first is only have sex with ONE person during the possible period of conception (and then only the mother could be sure). The second is with a paternity test. And you have to wait until after birth as no doctor is going to perform such a test just to conform paternity.

Apparently you are aware that she had another partner and I assume that she knows you are aware. So it should come as no surprise to her that you want positive confirmation.

As has been mentioned, pregnancy dating is an inexact science. It is all a matter of a mathematical formula, that depends on the date the egg was released from the fallopian tube, ovulation.

Since we don't know exactly when she ovulated, i.e. there was no camera in her fallopian tube, all we can do is formulate a best guess estimated delivery date using the woman's memory of her last period (LMP).

Gestational dating has a 2 week + or - window. That means that if a woman delivers her baby at 40 weeks (full term), she could be either 38 weeks or 36 weeks at delivery. Or even 40 weeks.

Unfortunately, you are going to have to have that DNA test done after the baby is born to determine who is the genetic father of the child.