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.
Thanks for your anwser. I really appreciate it. That idea will drive me crazy. But before claiming a DNA test, I should get some general information about this. Because from the website, the normal pregenancy period is 10 months, but we have been together only for 9 months. I do not want to do something stupid, but I really need the answer.
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Personal consultation(s) with a qualified medical professional is the proper means for diagnosing any medical condition.