Unless a baby is conceived in a petri dish, it is nearly impossible to determine conception. Many people mistakenly assume that conception occurs on the day a couple has sex. In reality, that is certainly possible, but it is also just as possible that conception actually happens a day or two or even more after. The reason for this is that sperm have the potential to live inside a woman for several days, possibly as many as 6-7 days. In addition, it is hard to know exactly when ovulation occurs. It is most common to determine a due date based on the first day of a woman's last period. Let's say that you were conceived on January 1. If your mom ovulated on the average 14 day of her cycle, than her period would have started around Dec. 18. A woman who began her period on Dec. 18 and than got pregnant, would be given a due date of September 24. Most full term babies arrive 2 weeks before to 2 weeks after a due date. If your due date was September 24 and you were born on September 13 it is very possible that the sex your mom had on January 1 resulted in your conception. However, if your mom had sex with another man in the last part of Dec. or early part of Jan., that man would have just as much possibility of being your father. Only a paternity test would be able to accurately determine who your father is.