Hi
I am very concerned about the accuracy of pregnancy dating ultrasounds.
I cannot remember my lmp but health professionals believe it to be 5.6.12
And my edd is 120313. I had my scan at 15 weeks and I am really worried because
People say that this is not accurate. I need to know that I am definitely 27 weeks because
I am really worried about it. How can they wrong? Can it be a month wrong or is that quite
Impossible? Did I definitely conceive in June?
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replied December 11th, 2012
Especially eHealthy
nadineh,

The following is taken from the MedScape website on “How accurate is ultrasound for dating gestation age?”

“Over the past 40 years, there has been a myriad of research studies exploring the safety, techniques, and benefits of ultrasound use in pregnancy. As ultrasound technology has evolved, so has its accuracy in later gestation, with current guidelines reporting a consistent 8% margin of error even well into the third trimester. This variance has evolved from years of study by several esteemed researchers who have sought the most accurate measurement parameters in determining gestational age and normal patterns of fetal growth…

However, this 8% margin of error, in terms of the number of days in error, will vary by gestational age of the fetus. When the margin of error is reported as a percent of the total, it is calculated against the number of days gestation verses the number of weeks. In other words, an 8% margin of error at 8 weeks' (or 56 days') gestation calculates to +/- 5 days; at 18 weeks' (126 days') gestation, this represents a range of +/- 10.08 days; lastly, at 28 weeks' (196 days') gestation, the margin of error continues to widen to +/- 15.68 days. Regardless of these inconsistencies in reporting, most concur that an ultrasound performed before 24 weeks' gestation establishes a more accurate EDD than relying solely on the LMP…

However, ultrasound's accuracy depends greatly on the skill of the person performing the examination and the quality of the images, not to mention the size of the patient and the fetal position.”


So, in general, the best way to determine gestational age of a fetus is with ultrasound evaluation. It has been proven to be much more accurate than using dates alone. However, there is a small margin of error, but it would not be a whole month off.

If you are concerned about the dates, discuss it with your OB/GYN.

Good luck.
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