Hello if the girl goes on her period on the 29th of march, and then I had protected sex with a condom on the 2nd of April, then she has a boyfriend that she claims to have unprotected sex with, and then ultrasound and pregnancy calculator says she was conceived on the 12 of April and she's is due on the 3rd of January, is there any chance this baby can be mine?

Plus the condom didn't pop.

Yes, there's a chance. The only way to know 100% is to wait until the child is born and then conduct DNA testing.

Yes there is a chance,however the unprotected boyfriend... in my opinion has a stronger chance.

DNA needed to be sure.