Hello,
Welcome to ehealthforum.
The EDD (expected date of delivery) is usually calculated by adding 267 days to the date of fruitful coitus.
As per your history, the EDD is 16th Jan 2010, it can vary about 10-15 days with the actual delivery happening either before or after this. This variation in the date also depends on the length of menstrual cycle the woman is having.
Do not doubt your wife's fidelity. If you do have reasons to believe that you might not be the biological father of the child, a detailed discussion with your doctor and DNA test will help.
Take care.