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How to Calculate Date to Conceive by Due Date

By Kathryn Rateliff Barr ; Updated April 18, 2017
The average pregnancy lasts 40 weeks.

The length of pregnancy is approximately 280 days. This is 10 lunar months or 40 weeks. Fewer than 6 percent of babies choose to be born exactly 280 days from their conception. Most babies are born with 10 days or so of their due date if nothing is done to artificially start labor. You can determine a conception date based on the average pregnancy length of 280 days.

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Planning Conception Date Based on Due Date

Determine when you would like the birth to occur. Count back 10 months before your preferred due date. This will be the conception month of your baby.

An average menstrual cycle is 28 days.

Determine the average length of your menstrual cycle. This is the number of days between the first day of one period to the first day of the next period. The average cycle length is 28 days. Your cycle may be longer or shorter than this.

Subtract 14 from the length of your cycle to find your average ovulation date. This number is also the average length of your luteal phase, the time period between the end of your period and ovulation. Conception usually occurs about 14 days before your period begins.

Mark the first day of your last period on the calendar. Using your cycle length, calculate how many cycles you will have before you reach your target month. Mark the specific day your period should begin that month.

Add the length of your luteal phase to that date to arrive at your probable ovulation date. You will want to time your sexual intercourse for the 24 hours before ovulation through the 48 hours following it. If all goes well, you should conceive during that cycle.

Things You Will Need

  • Calendar
  • Pen
  • Paper
  • Calculator


Illness, stress, hormone levels and traveling can all affect the regularity of your periods. Disease, stress, hormonal levels, alcohol intake, drugs, herbs, abnormal sleep schedule, traveling, holidays and late nights can all affect whether or not you ovulate during a cycle. Take care to avoid as many of these factors as you can during your target conception period.

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About the Author

Rev. Kathryn Rateliff Barr has taught birth, parenting, vaccinations and alternative medicine classes since 1994. She is a pastoral family counselor and has parented birth, step, adopted and foster children. She holds bachelor's degrees in English and history from Centenary College of Louisiana. Studies include midwifery, naturopathy and other alternative therapies.

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