How to Figure Out When You Conceived
Identifying the exact date when you conceived your baby can be difficult. If you weren't trying to become pregnant or were taking a laid-back approach, the only thing you may know about your cycle is that your period didn't arrive on time. While you can't identify your conception date with 100 percent certainty, you can use several different methods to help you pinpoint your likely date of ovulation.
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Instructions
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Identify the date your last period started and your average length from the start of one cycle to the next. If you have regular cycles that are approximately 28 days long, take the day your period began, which is considered cycle day one, and count ahead 13 days to find your likely date of ovulation.
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Determine the days you had intercourse since your last period began. If you only had intercourse a handful of times since your last period, narrow your possible conception date to within five days -- but more likely within two to three days -- after each time you had intercourse. Exclude intercourse that occurred 14 or fewer days before your positive pregnancy test.
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Have an ultrasound done during the first trimester to estimate fetal size. An ultrasound measurement of a baby's length from crown to rump is accurate to within three days, plus or minus, at seven weeks. The accuracy of an ultrasound measurement decreases with fetal age, and by the second trimester, it is only accurate within seven to 10 days, plus or minus.
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Calculate the date of conception based on your ultrasound measurement. An ultrasound estimate of fetal age -- like the one your healthcare provider uses -- considers the first two weeks from your period to when you ovulated as part of your pregnancy. If you measure seven weeks, five days on an ultrasound, then you conceived approximately five weeks, five days ago. To find your estimated date of conception, take your ultrasound estimate of fetal age, subtract 14 days and then count back the remaining number of weeks and days.
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Tips & Warnings
If you have long, irregular cycles, don't calculate your conception date based on your last period. You almost certainly ovulated much later than cycle day 14, so this method will not provide you with an accurate estimate of your conception date.
References
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