Conception, which occurs when a male's sperm fertilizes a female's egg, can usually only be estimated rather than definitely known. Calculating the conception date can help estimate a due date and is also helpful when trying to determine paternity or gestational age. According to The American Pregnancy Association, women with regular periods conceive somewhere between 11 to 21 days after their last period. This is because ovulation typically occurs in the middle of a complete menstrual cycle. It is important to remember that since sperm can live inside the female's body for up to five days, conception does not necessarily occur on the same day as intercourse.
Count 12 days after the date your last period started. This is the time just before most women ovulate, although your menstrual cycle may be different.
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Count forward an additional 10 days. Determine which days in that 10-day period you were sexually active. One of those days will be your conception date.
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Ask the ultrasonographer about the size of the baby during your first ultrasound. The measurements of the baby will indicate the gestational age, which will help you estimate a conception date.
Tips & Warnings
If you became pregnant through in vitro fertilization, you will be able to determine that the conception date was the date of the procedure.
Using a sonogram to determine the conception date is especially helpful if you do not keep track of your periods.
The most accurate way to determine a conception date is to be in the habit of consistently charting menstrual cycles, ovulation, and sexual activity before conception.
Not every woman has the same cycle length or ovulation period. It may only be possible to estimate your conception date.
