How reliable is first u/s? - Mothering Forums

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#1 of 9 Old 09-20-2005, 11:32 AM - Thread Starter
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I'm beginning to wonder... based on my first ultrasound at 8 weeks they set the due date 11/3. but based on LMP it would be 11/13. I know that 10 days shouldn't make that big a difference, but right now the only measurements that are right on to the 11/3 date is his femur. Is it possible that maybe his leg is just extra long and that the 11/3 isn't reliable? Right now, based on his other measurements, he would be only in the 19%ile and his head is about 2 1/2 weeks behind (1%ile). However, if you go by the 11/13 date then the measurements are better except for the fact that he has freakishly long legs. Personally, I like this scenario better.
I'm just worried that if they set the c/s by the 11/3 date, then he'll be born too early. I'd rather he bake as long as possible and fatten up!! The drs seem inclined to go with the 11/3 date since the first u/s is supposed to be very reliable, but if it is based on the femur measurement, then it could be wrong right?
(They want to schedule the c/s around 37 weeks.)

PS I have to schedule a c/s since he has spina bifida and will need surgery.
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#2 of 9 Old 09-20-2005, 12:03 PM
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I can understand your concern...I'd want to be sure of the dates too. I don't know how accurate an early u/s is, though I did know the exact conception date for my dd and her 1st u/s was in line with that date.

What does your fundus measure? That might offer some clue, too, though it can vary widely and may not correspond with your actual date either. If I were realy unsure and concerned, I might ask for an amnio to test for lung maturity before allowing them to deliver him.

Good luck to you.

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#3 of 9 Old 09-20-2005, 12:33 PM
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Dylan, my best friend is going through this whole issue right now as well. She knows the date of conception and the date of her LMP, but her 8-week ultrasound had her 10 days ahead of where she "should" have been at that time. As a result, her midwife changed her due date (ahead by 10 days) causing her to worry that they would pressure her to induce prior to when she was actually due. You can read about her saga here.

Most sources will say that ultrasounds done prior to 13 weeks are pretty accurate, but considering what I know happened with my friend, I am highly skeptical.

Ever-evolving mama to my beautiful Brynn, and my little dimple-face Noah .
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#4 of 9 Old 09-20-2005, 12:52 PM
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Early ultrasounds are supposed to be accurate plus or minus one week- meaning that from your ultrasound dating the due date range could easily be at the later end: 11/10. And as you get further along, they become less and less accurate- plus or minus two (or more?) weeks. And some babies are just built different that the average- a long femur isn't unreasonable. At certain gestational ages different growth markers are more relied upon for dates, I think- but I don't know which ones and when...

You should talk to your caregivers about your concerns, especially if a c-section is planned for 37 weeks. At that stage dates off by even a week could easily give you a premature baby- 35 1/2 weeks by your LMP date. Waiting a little bit later (even a week or week and a half) for the c-section might ease your mind, but I don't know how it would affect the plans for surgery. Good luck!

"Guess what? It's a magical world. And when I sing, my songs are in it."
Madly in love with my 7 and 4 year old daughters

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#5 of 9 Old 09-20-2005, 12:54 PM
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I know our conception date also & our early US was dead on. I dont know the average accuracy of them though
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#6 of 9 Old 09-20-2005, 02:01 PM
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Generally, u/s in the first trimester is considered extremely accurate. Both times I've conceived I've known my conception date without at doubt, and the dating u/s at 6 weeks was correct to the day.

Most dr/mw practices will let you "play" with the due date a bit, and go by either the u/s or LMP, whichever works best for you - especially if you are trying to avoid an induction, you can usually go with the later date.

Mama to DS (05/04) and DD (11/05), married to a wonderful DH.
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#7 of 9 Old 09-20-2005, 02:29 PM
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I have also heard early u/s are very accurate, mine was to the day.

However if you are concerned you should definitally speak with your care provider. A concerned mama = a concerned baby, and nobody wants that.

Your concerns are valid, I would really bring it up.

Good Luck!
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#8 of 9 Old 09-20-2005, 04:36 PM
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I had an ultrasound at 12 weeks and one at 19 weeks and both agreed exactly - to the day - with my own sense of when I conceived. But I don't think that helps predict my due date at all! I'm telling people the baby could come any time 5-10 weeks from now. (I'm 32 weeks.)

Postpartum doula & certified breastfeeding educator, mama to an amazing girl (11/05) and a wee little boy (3/13).

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#9 of 9 Old 09-21-2005, 06:31 AM
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There's a lot of discussion on the homebirthUK list about the whole 37/38 week issue- which is based on the fact that although WHO consider 37 weeks to be "term", several studies have been done recently that have shown that babies born by elective c-section at 38 weeks do better than those born at 37- which, obviously, is even more important to bear in mind given that you know to expect problems.
Links to studies are at the bottom of this page:
In your situation, I'd probably ask that the surgery be delayed until you're 39 weeks pregnant by scan dates, 38 by yours, unless baby's growth slows down further. Crista's idea of an amnio, in the circumstances, seems like a good one too.

Helen mum to five and mistress of mess and mayhem, making merry and mischief til the sun goes down.
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