Discrepancies between ultrasound and calculated due date
How could a 4-week discrepancy in due date be explained between calculated due date and the due date a doctor calculates after the first ultrasound?
For example, if we received the 12. of November as due date from our doc after the first ultrasound, this would put us in 7th week.
However, if we use the day of the first last period (e.g. 14. of January) the due date would be: 20. of October.
So how could such a discrepancy be explained?
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Ultrasound is an estimate: measure the size and development of the fetus, and compare it to standard tables of development to figure out how far the pregnancy has progressed. In the first trimester, length measurement alone is used.
A difference of 23 days at either 7 or 10 weeks of development is far outside the normal range of variation. There are many things that could cause this, ranging from a bad measurement by the ultrasound technician, to a missed period causing conception to be later than believed, to development abnormalities. You'll need to consult with your doctor to find out which it is.
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