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Which ultrasound do I trust for gestational aging?

bcr3926bcr3926 member
in 3rd Trimester
I had my first ultrasound on July 1st and could only see a 9mm gestational sac but no yolk sac or fetal pole. My second ultrasound was on July 15th, everything was visible this time and they measured the baby at 7w1d. I went for another ultrasound on September 30th (18w1d) and the baby was measuring 19w2d (1w1d big). Today, at 30 weeks, I had my third trimester ultrasound/growth scan. The baby was measuring at 32w5d (2w5d big) and weighed 4.5lbs. My due date is March 1st but I'm wondering if it's possible that I'm actually further along than the doctor's originally thought and with so many different measurements which one do I trust?
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Re: Which ultrasound do I trust for gestational aging?

  • jmathi0149jmathi0149 member
    edited December 2015
    Keep in mind EDD can be +/- 2 weeks so you could be looking at as early as late February or as late as mid March.
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  • ashhsaashhsa member
    The very first one is often most accurate. I've been told multiple dates too.
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  • bcr3926bcr3926 member
    Also, if I was actually 2-4 weeks further along, wouldn't the "5 week" ultrasound have shown more than just a yolk sac? Is it possible for an ultrasound that early to be off by that much?
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  • SarahFoley725SarahFoley725 member
    My doctor told me "all babies are the same size at 8 weeks....20 weeks, not so much" The earlier scans are most accurate but even those are +/- a week or something like that.
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  • g8trkimg8trkim member
    Earlier ultrasounds are the most accurate. Plus, anticipating that your due date is actually earlier is probably only going to set you up for disappointment.




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  • fourtsixand2fourtsixand2 member
    Another poster said the estimated due date was +/- 2 weeks, but I personally have never heard that.  I've heard it was +/- 3 or 4 days.
    In my opinion, I'd go by your estimated due date.  I'm also having to go off my estimated due date as I have never had a regular period and quite literally had no idea when I conceived.  In fact, I was told I was 12 weeks pregnant when I thought I was only about 7 weeks, so I'm going with the first date I was told.  
    Good luck!  
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  • NicolewinsbingoNicolewinsbingo member
    Earlier ultrasounds are more accurate, usually to within a few days of the actual truth (if they say 8w2d, you may actually have been 8w4d but close enough). Later scans have a wider margin. All that said, a due date is just an estimate anyway; your baby will be born when he's ready.
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  • mullenixjmullenixj member
    OP I had the same issue! I went to just a regular imaging place for the 1st US and should have shown that I was 7 weeks along, but had the same thing as OP and was told it looked only 5 weeks, had to go back 2 weeks later for another. Was told then that I was 7 weeks and due Jan 22. Based on science, it would be impossible for my due date to be that late but the tech told me I must have my days mixed up. Went to my first OB appointment and she did an US that put my due date at Jan 17 (which fits the time line of when I could have conceived) but they wouldn't change the EDD because it was less than a week. My doctor said I was the 2nd person that day that went to the same imaging center and she did not agree with the EDD. The 20 week US also put my EDD at Jan 17. She said next time I get pregnant to make sure I go to a specialized maternity US center for the initial dating scan.
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  • huskypuppy14huskypuppy14 member
    mullenixj said:
    OP I had the same issue! I went to just a regular imaging place for the 1st US and should have shown that I was 7 weeks along, but had the same thing as OP and was told it looked only 5 weeks, had to go back 2 weeks later for another. Was told then that I was 7 weeks and due Jan 22. Based on science, it would be impossible for my due date to be that late but the tech told me I must have my days mixed up. Went to my first OB appointment and she did an US that put my due date at Jan 17 (which fits the time line of when I could have conceived) but they wouldn't change the EDD because it was less than a week. My doctor said I was the 2nd person that day that went to the same imaging center and she did not agree with the EDD. The 20 week US also put my EDD at Jan 17. She said next time I get pregnant to make sure I go to a specialized maternity US center for the initial dating scan.
    Why would a 5 day difference matter, and how is the 22 impossible and the 17 is not? That makes no sense. Even if you were temping, you can have a slow rise or something else, so unless you had an ultrasound and knew exactly when you ovulated, you don't know that the 22 is impossible. Sperm can live in the bod for up to 5 days, so if you know when you had sex, that might not be the day you conceived.  Unless I'm missing something entirely. Some times implantation happens later or earlier so even if you knew exactly when you conceived, it could still be a couple days difference.



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  • cg0112358cg0112358 member
    brittanyaddison said:

    Also, if I was actually 2-4 weeks further along, wouldn't the "5 week" ultrasound have shown more than just a yolk sac? Is it possible for an ultrasound that early to be off by that much?

    If you were 2-4 weeks further along than 5 weeks your u/w would have shown a baby with a heartbeat.

    As for the u/s, yes than can be off by quite a large margin. That being said I really wish u/should techs and doctors would present the information more clearly. Later u/s measure the size of the baby, not the development. Your baby is measuring larger, that does not mean he/she is developed more and therefore "older." At my 38-week appointment with DD1 I measured a little over 2 weeks ahead and was told the baby weighed over 8 lbs. Two weeks later I delivered what should have been an over 9 lb baby based on the u/; she was just over 8 lb which means the u/s estimate was,off by 2 weeks and DD1 was actually right on track in terms of size.
    DS1: 8/3/10, DD1: 8/17/13, DD2: 8/13/15
    Twins lost to due to partial molar pregnancy: September 2011 
    ~~PAL, PgAL Always Welcome~~
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  • mullenixjmullenixj member
    I know when my husband and I had sex and that sperm can live up to 5 days so based on that, it does not add up for the 22nd as my due date. The 17th is actually on the edge of that timeframe. Based on my last period, my due date would be Jan 10, which actually fits the conception window the best.
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  • delujm0delujm0 member
    In the long run it doesn't even matter. It's just an estimate. +/- 2 weeks is totally normal. Earlier ultrasounds are more accurate. I temped and knew ovulation date. I measured 3 days early at my 8 week scan. I delivered at 11:55pm the day before the due date determined by that scan. So it was highly accurate. SIL just delivered 3 weeks early. Her baby is fine. There's a huge range, it doesn't need to be dead on accurate. The baby will come when it's ready. It doesn't care about the due date.
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