Thanks for the query.
As on 21st Feb, if your LMP was November 20th then you are 13 week and 2 days pregnant by LMP.
Considering that you had regular cycles, your ovulation should have been around 4-6th December. so the sexual encounter closest to the ovulation day is most likely to be the date of conception. The possibility that the man on December 6 is the likely father of your child. However, the confirmation can be made by amniotic fluid DNA sampling from foetal cells and getting a paternity test done.
I hope I have answered your query in detail.
Wishing you a safe pregnancy.
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