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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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    We had a similar experience, my wife had been pregnant four weeks longer than expected. The dating on the first scan is based solely on size, and up to a certain point, there’s basically no variation in size, so that’s really accurate. – AJFaraday Mar 21 at 22:05
  • 4 weeks longer makes sense, after all its possible to get a period while pregnant, but 4 weeks shorter? How could that be explained other than, the embryo stopped growing 4 weeks ago. – user1721135 Mar 21 at 22:22
  • Well, they’d know if they had stopped growing, there’d be no heart beat. – AJFaraday Mar 22 at 8:57
  • @AJF: that's not very reassuring in this particular case if you read the entire question. – David Hedlund Mar 22 at 12:16
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    Hi, welcome to the site. Unfortunately medical questions are best asked to your doctor and not here; while I think the broad question is ok, it’s clear this is rooted in a specific medical question and that needs professional advice. Good luck! – Joe Mar 22 at 13:06
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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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  • could it also be that the ovulation happened so much later than calculated? – user1721135 Mar 22 at 15:11
  • @user1721135, that's also a possibility. – Mark Mar 22 at 18:18
  • Nice answer, +1. I bolded the most important takeaway, though. I hope that's ok. Feel free to roll back if it's not ok. – anongoodnurse Mar 23 at 22:41

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