Lassies, here is the deal, you are feeling guilty. And don't want to tell, so the stress increases. So you let it settle on being obsessive about the date, even when your common sense tells you that is not really the problem, because worrying about dates is easier than worrying about your integrity.
A guy went to a psychiatrist for some therapy because he had an obsessive feeling that he was a piece of corn being pecked by giant chickens. He worked with the therapist for many weeks and finally finished his therapy. As he was leaving, the doctor said, "So, now you know you are not a piece of corn being pecked by giant chickens, right?" and the guy said "Yes! ... But are you *sure* the chickens know?" Don't let your anxiety settle on something so obviously not the real issue.
Deal with the stress for what it is, and if you cannot bring yourself to tell your beloved what you did, atone in other ways. Never cheat (again), and make him him the happiest daddy in the world from now on out. A lot of guys would actually take that trade if it was offered to them -- ignorance of something that they should by rights know, in exchange for a lifetime of future happiness.
im pretty much in the same boat as you, my due date is a 5 week 3 day difference from the time I had sex with both different partners, I had my period and everything I still get worried. I dont understand why. I know its like not a chance unless my dates would randomly change for some reason but I had an ultrasound 8 weeks to the day. soooo idk. im a freak.
Mr. January 27 would not have anything to do with it. Listen to your doctors.