Hi,
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If your LMP was Sept 7th, 2011, then your probable conception date would lie between Sept 17th - Sept 25th, 2011, with ovulation occurring around Sept 21st, 2011 (calculation based on menstrual cycle length of 28 days). The intercourse closer to your ovulation time would result in pregnancy (after ovulation, egg is alive for next 24-48 hours to be fertilized). Since there was no period between your two different sexual encounters, it is difficult to comment on the paternity. Consider requesting for dating scan to arrive at your probable conception date. The exact results can only be made out after a Paternity test. Visit your doctor/gynecologist for further assistance and clarification.
Take care.