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?

  • 1
    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, 2020 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. Mar 21, 2020 at 22:22
  • Well, they’d know if they had stopped growing, there’d be no heart beat.
    – AJFaraday
    Mar 22, 2020 at 8:57
  • @AJF: that's not very reassuring in this particular case if you read the entire question.
    – dxh
    Mar 22, 2020 at 12:16
  • 1
    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, 2020 at 13:06

1 Answer 1


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.

  • could it also be that the ovulation happened so much later than calculated? Mar 22, 2020 at 15:11
  • @user1721135, that's also a possibility.
    – Mark
    Mar 22, 2020 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, 2020 at 22:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.