It is not certain that you can be born from the time of intercourse to 266 days, only that you are on your due date. The due date is calculated from the first day of the first period and 280 days after that. If your period is normal, i.e. 28 days, ovulation occurs on the 14th day of your period. If you have intercourse at this time and become pregnant, you will be in labor in 266 days. It is normal for a baby to be born three weeks before or two weeks after the due date, i.e. a baby born between 37 and 42 weeks of pregnancy is within the normal range.