One simple and common method for calculating your delivery date is as follows:

  • Mark down the date when your last period started.
  • Add seven days.
  • Count back three months.
  • Add a whole year.

For example, if the first day of your last period was September 9, 2000, adding 7 days will make the date September 16, 2000. Counting back 3 months results in the date of June 16. Finally, adding a year yields an estimated delivery date of June 16, 2001.

The above strategy is based on the assumption that conception occurred 14 days after the start of your last menstrual period. Also, it's important to remember that no matter what "pen and paper" calculations you use to estimate your delivery date, these methods are just that – estimates. Most babies are born between 38 and 42 weeks (the normal pregnancy is considered to be 40 weeks in length counting from the first day of your last menstrual cycle) and only a small percentage of women actually deliver on their estimated due date.

Your doctor can use other methods to provide you with your delivery date, such as conducting an ultrasound examination in the first trimester. A physical exam also can be done.