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Thread: confused about ultrasound dating scan

  1. #1
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    Default confused about ultrasound dating scan

    So ladies, can you help me clear this up?

    the first day of my last period was april 29, which if calculating using the last period would make me 7 weeks 2 days. I had a dating ultrasound done today and the tech said i was 8 weeks along... how can this be if it hasnt been more than 7 weeks since my last period? soooo confused? it doesnt make any sense....seems like i am farther along than i should be?
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    They make an estimate based on how big the baby is I believe. When I was pregnant with DD they initially said she was due December 15th, then they changed it to the 18th when we went to a later ultrasound. She was born on December 10th. I wouldn't be too worried about it personally.

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    Don't worry about 5 days difference, they may change your due date a few times throughout your pregnancy with other ultrasounds. The tech shouldn't have said anything anyways, your Dr will have the results in a couple of business days (Tuesday) call and ask your Dr questions next week. I am confused as why they would do a dating ultrasound if you know exactly when you had your last period.

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    When I had my first u/s at 7 weeks I was dated +/- 4 days. The earlier the u/s the more accurate, so they shouldn't change your dates based on later u/s, as the +/- will get bigger each time. Now adays they don't tell much at the u/s, but when I was in Windsor 11 years ago and had spotting I learned a lot at the u/s and got to see everything.

    While your last period was 7w2d ago, that is based on the average assumption that you ovulated and got pregnant exactly 14 days later. If you ovulated 9 days after your period then the u/s date is accurate. Also, there was likely a +/- (unless u/s machines are so exact these days), so it could be that you ovulated 11 days after your period if you were 8weeks +/- 2 days.

    Basically, it is your exact day of conception PLUS 14 days, so if you ovulate earlier or later than that "average" then it can make the number seem bigger or smaller.
    Last edited by melissawilliam; 06-19-2015 at 08:12 PM.
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    I also seem to cook my kids fast too.

    My last one I was using OPKs and by US they estimated my EDD to March 11th, by ovulation my EDD was March 16th. She came March 11th.

    My middle was dated July 8th by US, and July 16th by ovulation. She came July 5th.

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    Quote Originally Posted by Pomegranate View Post
    DI am confused as why they would do a dating ultrasound if you know exactly when you had your last period.
    I had a dating US with number 2 even though I knew my LMP. The month I got pregnant was pretty brutal. my 2nd as my FIL had passed away after a long battle with cancer, I had H1N1, and my DH had surgery all around the same time that month.. then AF was late... I had a few negative HPTs and finally had a positive. I didn't get a positive test until like cycle day 38-40 or something.... and I'm a 29 day cycle person. So I think the stress of the month delayed ovulation... MW thought it would be best to know a more approximate date as I was attempting a vbac and she wanted to avoid an induction at 42 weeks if I wasn't actually 42 weeks.

    If her cycle was a bit off that month, or if she was normally irregular going with LMP is not an ideal way to come up with the EDD.
    Last edited by a_bun_in_the_oven; 06-19-2015 at 08:22 PM.
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    I would just go by your cycle if you are regular. The doctors and techs insist on going by the ultrasound for some reason, my midwife went by my cycle. In reality 5 days does not make that much difference, all my children were two to four days over.....everyone is different. Babies come when they are ready. I would give the advice to anyone not to let them induce you! Unless there is a medical reason of course.
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