Hi, Thanks for posting in HCM. I understand your concern. The exact date of conception cannot be known precisely. For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period. Most women do not know the exact date of conception because it can be challenging to know exactly when ovulation would have occurd. However, it can be calculated indirectly by knowing the expected date of delivery (EDD). EDD can be known by two means: 1. Using Naegele s rule in which we use the formula based on the first day of last menstrual period (LMP). 1st Day of LMP + 1 Year and Subtracting 3 months and then adding 7 days. 2. By means of ultrasound scan based on fetal biometry. Pregnancy Conception Calculator can help to find out the pregnancy conception date based on the EDD. Correspondingly, it would also give the range of days during which sexual intercourse might have led to the conception, based on the factor that sperm can live inside a woman for three to five days. You can do know it by visiting website such as: http://www.calculator.net/pregnancy-conception-calculator.html However, precision would only improve if your cycle was for 28 days and regular. In case, it is inconclusively, it would be advisable to undergo DNA fingerprinting testing after birth to know who could be the biological father. Hope the information provided would be helpful. All the best.