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The person with whom you had intercourse during your fertile period (ovulation or egg release time) will be the father of the child.
How many days duration menstrual cycle do you have? For a menstrual cycle of 28-30 days, ovulation time lies between 8th-16th days, taking day 1 as first day of bleeding.
For best clarity, further tests can be done to know who the father of the child is. Discuss about getting these done with your healthcare provider/gynecologist.
All the best.