My guess is that Michigan follows the same legal presumption, as do most of the states:
If a child is born or conceived during marriage there is a rebuttable presumption that the husband is the biological father.
If the estranged wife will not concede and thereby satisfactorily convince the court that it is not yours, but another named man’s child, then you will need to request the court to order genetic testing to be performed once the child is delivered.
You definitely need legal help with this. There is too much at stake financially for you to go it alone.
For instance your attorney should do some aggressive pre-trial discovery to develop whatever sexual relationships she has had, with whom and when, etc.
Eighteen years of supporting someone else’s child will cover a lot of attorney fees. Besides if the child is yours, you would surely want to acknowledge it.