My wife is pregnant, and I Worried about AWALT just because. I highly believe there has been no cheating, but I want to check DNA just because after the baby is born. What's the best method? 23andme?

Also, how accurate will the doctors predict the due date? She was away on a trip with her friend, and got pregnant right after or close to that day from what I counted back to. I'm worried that due to the closeness, it may have happened during the trip. I'm a paranoid mofo though. Thoughts?