Reverse Due Date Calculator
to Calculate Conception Dates

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The online reverse due date calculator on this page will instantly estimate the date and day of the week your baby was conceived on.

This free online conception calculator will determine the conception date -- including the day of the week -- based on the selected month, day, and year of your pregnancy due date.

Plus, unlike other conception calculators, this calculator includes selectable dates dating back 100 years, which means it can also be used to calculate conception dates and days of the week for any dates of birth occurring within the past 99 years.

Want to know what date and day of the week you were likely conceived on? Simply select your birthday (or due date if you were born premature or late) and click the "Calculate" button.

Note that if you want to calculate what date your baby is due to be born on, please visit the Pregnancy Due Date Predictor -- which also includes How to Calculate Due Date based on Naegele's Rule.

With that I invite you to use the reverse due date calculator to estimate the date and day of the week your baby was conceived on (or any one else for that matter).

Reverse Due Date Calculator
Calcy sign introducing Reverse Due Date Calculator


Instructions: Select the month, day, and year your baby is due, and then click the "Calculate Conception Date" button.

Mouse over the blue question marks for a further explanation of each entry field. More in-depth explanations can be found in the glossary of terms located beneath the calculator.

Note that this conception calculator generates conception dates based on averages and therefore can only be considered to be estimates.

Important! My expertise is creating online calculators based on known formulas, not obstetrics, gynecology, or paternal law. So if you have questions related to any of those fields, please seek out the advice or counsel of a qualified physician or attorney.

Help Pregnancy due date (or birthday):
Help Estimated conception date:
Help Last period date:

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Reverse Due Date Calculator Glossary of Terms

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Pregnancy due date (or birthday): Select the month, day, and year your baby is due. Note that you can also select any birthday to calculate when you or someone else was most likely conceived.

Estimated conception date: This is 266 days (38 weeks) subtracted from the selected pregnancy due date (the average human gestation period per Naegele's Rule).

Last period date: This is 14 days prior to conception (assuming ovulation occurred on day 14 of your menstrual cycle).

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