DNA test is the only way to know. Its always possible its not yours...but it seems your questioning if she was faithful. If you have any doubts they have paternity tests you can take BEFORE the baby is even born. You may want to look into that to ease your mind. Just to give you a time frame...i concieved 10/25 found out 11/5 and im due 7/17...so it could really go either way...there is no way to pinpoint the actual day or moment...
Ultrasounds give you an estimate due date. They have given me 2 different due dated some go by your last menstrual cylce others by what they think was you conception day. This got my bd confused too n he didnt believe me so I made him as the Doc and he explain things to him. Also all babies are different and some are consider "normal" some measure smaller and some a measure bigger. But they are not always 100%
I'm sorry I can't answer your question but I'm sure someone else can. You could ask for a DNA test after baby is born but that might offend her if she's been faithful. Hope you get some good answers and good luck!