cococandy:
Estimated gestational age usually adds two weeks to the actual age of the fetus. So using your mathematical skills may lead you wrong.

However if you have genuine doubt, take her up on the DNA offer.
You'll have to live with the result.
Negative = heartbroken.
Positive= a wife who will forever make you miserable for doubting her faithfulness.
Not doing the test = consistent doubt about the real father of your child.

There's really no easy way out for you here. Just chose the option that you can live with the results.

I wish you and your new family the best of luck.

the wife created reasons beyond reasonable convictions to doubt her faithfulness, even if the child turns out to be his, the actions of the wife made him doubt the paternity in the first place.