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You are correct that the doctor calculates the due date by adding 40 weeks from the first day of a womans last period. Actual conception typically occurs 14 days before the first day of the expected (missed) period. Keep in mind that this is all based on a 28 day cycle. Your girlfriends dates may vary by a few days if her cycle is longer or shorter than 28 days. If you are the father, her last period would be 2 weeks prior to your last time you were together, which was 6/27 (approx 6/13).
Why did she not tell you until just a few months ago? Are you sure she is giving you the true due date? If you want to check, it is usually printed on the ultrasound picture which you can request to see without being accusatory.

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