It's important to find out your due date. Why? So you and your doctor can plan for proper care of you and your baby. Your doctor uses a method similar to this.
Since your last period was on , your baby's birth date is on or near .
Doctors have found that a baby stays in the womb for about 266 days. This is from the day you became pregnant. It is also 280 days after the first day of your last period. Your normal cycle can be 21 to 35 days. But your doctor uses 28 days when figuring out the due date. Your eggs become fertile on day 14, with a 28-day cycle. So the egg could not be fertile until two weeks after the first day of the cycle.
Most doctors know that many things affect a due date. Your doctor can use other ways to help find your due date.
March of Dimes, Calculating Your Due Date. Accessed at: http://www.marchofdimes.com/pregnancy/calculating-your-due-date.aspx