Determining the conception date of a child is more of an estimation than an exact calculation. This is partly due to ovulation. Most women with a consistent 28-day cycle ovulate 14 days after the start of their previous period, notes MayoClinic.com. Around this time is when you're most fertile and likely to conceive. The other reason that the conception date is more of an estimation is gestation. Though gestation averages out to about 40 weeks, the American Congress of Obstetricians and Gynecologists explains that women typically give birth anywhere between 259 and 294 days after conception, which is 37 to 42 weeks.
Take the child's birth date and count back 40 weeks. If your child was born on the 23rd of August, conception is placed somewhere around the end of November to the beginning of December.
Determine the first day of your last menstrual period within this allotted time. Using the same example, the first day of your last menstrual period is most likely mid-November, roughly November 15th to 17th, if you have a regular 28-day menstrual cycle.
Count out 14 days from the date of your last menstrual period to determine the probable date of ovulation. With an August 23rd birth date, ovulation most likely occurred on November 30th.
Count back four days as well as forward four days from the ovulation date to determine the range of conception. If November 30th is the probable date of ovulation, conception could have occurred anywhere from November 26th to December 4th.
Think back to those days when you had sex to determine the exact date of conception.