How to work out conception date from birth date

Updated March 23, 2017

Many people would like to work out the conception date of a baby from the baby's birth date, but aren't quite sure how to go about doing this. Luckily, it is an extremely simple and straightforward process that can be done in just a few steps. However, it should be pointed out that this method of working out the conception date from the birth date is not entirely accurate, as the length of conception can vary from woman to woman and even from pregnancy to pregnancy. However, if just a general indication of the conception date is needed, the following method should adequately serve that purpose.

Go back 266 days from the birth date of the baby, using a calendar. Although the average gestational period is said to be 280 days, this 280 days starts from the date of the woman's last menstrual cycle, which includes the two weeks before conception occurs. Therefore, subtracting those 14 days from 280 will equal 266, which will allow you to come closer to the conception date rather than the date of the last period.

Use that date to create a range of when conception most likely occurred. As mentioned earlier, it is difficult to pinpoint an exact conception date since the time of conception has so many unknown variables. However, adding about a week to both the beginning and end of the date that is 266 days before the birth date should give you a fairly accurate estimated range of when conception occurred.

Try to remember or determine the days during this date range where sexual intercourse took place. This will help you pinpoint actual dates where conception may have occurred. However, be sure to remember that sperm can survive for up to 5 days after intercourse, so the actual date of conception may be up to 5 days after intercourse took place.


If a more precise or accurate conception date is needed, it may be more useful to take measurements of the foetus in early pregnancy through an ultrasound. According to the American Pregnancy Association, one of the most accurate ways of determining the date of conception is by calculating the first day of the woman's last menstrual period and confirming it by measuring the foetus through an ultrasound exam sometime between the 8th to 18th week of pregnancy.


Any method of calculating a conception date, even by ultrasound, may not be entirely accurate as there are many variables that need to be taken into consideration. A woman with irregular periods, a baby that grows more quickly or slowly than average, or a baby with a shorter or longer gestational period are just a few things that may skew the results.

Things You'll Need

  • Calendar
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About the Author

Elizabeth Wolfenden has been a professional freelance writer since 2005 with articles published on a variety of blogs and websites. She specializes in the areas of nutrition, health, psychology, mental health and education. Wolfenden holds a bachelor's degree in elementary education and a master's degree in counseling from Oakland University.