Children's Health and Wellness

Calculating a Due Date

A typical pregnancy lasts, on average, 280 days, or 40 weeks — starting with the first day of the last normal menstrual period as day 1. An estimated due date can be calculated by following steps 1 through 3:

  • First, determine the first day of your last menstrual period.

  • Next, count back 3 calendar months from that date.

  • Lastly, add 1 year and 7 days to that date.

  • Due Date Calendar
    Click Image to Enlarge

    For example: Your last menstrual period began on September 9, 2010. Counting back 3 calendar months would be June 9, 2010. Adding 1 year and 7 days would bring you to June 16, 2011, as your estimated due date.

    This 3-step method is referred to as Naegele's Rule and is based on a normal 28-day menstrual cycle. Therefore, dates may have to be adjusted for longer or shorter menstrual cycles. 

    You may also estimate your delivery date by using the steps 1 and 2 and the chart:

  • First, locate the first day of your last menstrual period (in black text).

  • Next, note the date directly below. This is your estimated date of delivery (in blue text).

  • Print Source: Youngkin, E. Women's Health: A Primary Care Clinical Guide. 2013, ed. 4, ch. 19.
    Online Source: Calculating Conception, American Pregnancy Association
    Online Source: Prenatal Development: How Your Baby Grows During Pregnancy, American College of Obstetricians and Gynecologists
    Online Source: Calculating your due date, March of Dimes Foundation
    Online Editor: Geller, Arlene
    Online Medical Reviewer: Finke, Amy, RN, BSN
    Online Medical Reviewer: Sacks, Daniel, MD, FACOG
    Date Last Reviewed: 8/10/2015
    © 2000-2017 The StayWell Company, LLC. 800 Township Line Road, Yardley, PA 19067. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.