I think you should get a better OCD therapist. You must have been told by now that giving in to OCD-related false messages (that your logical mind knows are not correct), simply strengothens the OCD. Then you need more and more reassurance, and even then it doesn't work. If your therapist is not telling you these things, it is important to find a better therapist, because you are going to be a mommy, and you can't indulge in the luxury of such false thinking.
Your conception date is November 27 according to three early ultrasounds. Ask your doctor if you like, but obviously only you can reassure you.
You are thinking "what if" doctors looked at ultrasounds three separate times and did not notice that the baby was a month older than it should be for the due date?
I feel like I'm overthinking this but the "What ifs" is really weighing on me.