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Dr. Andrew Rynne
MD
Dr. Andrew Rynne

Family Physician

Exp 50 years

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Can Exact Conception Date Predicted From The Last Menstrual Date?

I had sex on feb14th 15th and also on feb28th unprotected and im pregnat now my last lmp was in the beginning of Feb I get my period every month for 7days but get it on different days my question is when was the day I most likely got pregnant I m about 34 weeks 4 day but ultrasound sound says 33weeks 4daya
Wed, 15 Jun 2016
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Infertility Specialist 's  Response
Hello dear, thanks for posting your query.
It is nearly impossible to predict the exact conception date so accurately when menstrual cycles tend to vary. The day of ovulation can be predicted from date of last menstrual period( and hence date of conception ) when cycles are perfectly regular and also from very early ultrasound with an error of +/- 5 days. The gestational age calculated according to ultrasound scan usually corresponds to the gestational age calculated according to last menstrual period when cycles are accurate and it is considered acceptable when both vary about 5 days in the first trimester.
Whereas it is acceptable when the discrepancy is a week and 1.5 weeks in the second and third trimester respectively.
Ultrasound findings should not be considered for calculating gestational age in the third trimester as the error will be too high and unless the woman doesn't remember her last menstrual period and no previous scans have been done to know her gestational age.
Your scan findings are perfectly normal and they are corresponding and you must use your LMP to calculate your estimated delivery date or early trimester dating scan. A discrepancy of one week in the third trimester scan is absolutely nothing to worry about and the growth of the baby seems just fine ( though the measure of growth is AC in percentile ) seen in third trimester. Do relax and enjoy coming up motherhood!
Good luck!
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Can Exact Conception Date Predicted From The Last Menstrual Date?

Hello dear, thanks for posting your query. It is nearly impossible to predict the exact conception date so accurately when menstrual cycles tend to vary. The day of ovulation can be predicted from date of last menstrual period( and hence date of conception ) when cycles are perfectly regular and also from very early ultrasound with an error of +/- 5 days. The gestational age calculated according to ultrasound scan usually corresponds to the gestational age calculated according to last menstrual period when cycles are accurate and it is considered acceptable when both vary about 5 days in the first trimester. Whereas it is acceptable when the discrepancy is a week and 1.5 weeks in the second and third trimester respectively. Ultrasound findings should not be considered for calculating gestational age in the third trimester as the error will be too high and unless the woman doesn t remember her last menstrual period and no previous scans have been done to know her gestational age. Your scan findings are perfectly normal and they are corresponding and you must use your LMP to calculate your estimated delivery date or early trimester dating scan. A discrepancy of one week in the third trimester scan is absolutely nothing to worry about and the growth of the baby seems just fine ( though the measure of growth is AC in percentile ) seen in third trimester. Do relax and enjoy coming up motherhood! Good luck!