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I would need to know the following:
Do you have regular cycles?
How early did you have an initial ultrasound - how many weeks were you at that scan?
Thank you for the followup information. The due date should have been based on the earliest ultrasound, so, without knowing much more about your story, I would date the pregnancy based on the scan at 8 weeks giving you a due date of 2/22. Based on this due date, conception occurred 6/1 (+/- 5 days). So the window within which you conceived was 5/27 - 6/6. It is almost certainly GuyB. Guy A is just way to far before when conception would have occurred given the findings at the time of the ultrasound. Also, you had intercourse with Guy A at a time in your cycle when you are not fertile (assuming regular cycles).
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