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how accurate are 3rd trimester ultrasounds for determining the "age" of baby?

based on what I was told at 8 weeks I should be 38 weeks pregnant now. my midwife did a fundal height and said I'm 3 weeks behind. she wants me to get an ultrasound done. I thought the 8 week ultrasound was more accurate for telling how far along I am. what is she looking for?

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Asked by Anonymous at 12:14 AM on Jul. 18, 2009 in Pregnancy

Answers (4)
  • I think they would be looking at the organ development that late in the pregnancy, like how far the lungs are developed. Maybe the size of the baby too. I also think the earlier U/S is more accurate because they are changing/growing so rapidly it's easier to tell a 6 week baby from a 9 week baby. You can't really tell much difference between 36 and 39 weeks.

    Answer by OwensMama824 at 12:30 AM on Jul. 18, 2009

  • She's looking for growth restriction. My son had this (IUGR). I started measuring small at 35 weeks. Turns out my placenta wasn't working properly and my son wasn't growing properly. I had him 4 weeks later (39 weeks) he weighed 5 lbs 4 oz. That is most likely what she is wanting to RULE OUT. Don't stress about it. It is typical to check the growth when the mom's growth slows down.

    Answer by mnt_2_b_mommy at 1:42 AM on Jul. 18, 2009

  • she might just want to check his growth. but, your baby could have dropped and that can change the measurement. i didn't go by the measurements. mine always changed, i was 2 wks ahead, then 2 wks behind, then right on target. so, i just started to ignore it.

    Answer by mommytobobby at 10:02 AM on Jul. 18, 2009

  • The first ultrasound is more reliable, late ultrasounds are not reliable ways of determining the babies size at all. There is no reliable way to determine that until the baby is born.

    Answer by Cynthje at 10:20 AM on Jul. 20, 2009

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