Accuracy of estimated fetal weight?: Hi All Am 35+2wks... - NCT


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Accuracy of estimated fetal weight?

BristolLady profile image

Hi All

Am 35+2wks and my growth scan today estimated a jaw dropping fetal weight of 7lb 13oz! (now!). Knew we were expecting a big baby but that was way higher that we could imagine, with >95% abdominal circumference.

But I also know these estimates can really over egg it (a friend’s actual baby weight at 39.5 wks was a good bit less than the estimate at 37wks).

Just wondered others experiences of high estimate weights compared to actuals?

We’re back in a couple of weeks for a further scan and possibly a cervical sweep/contemplation of a planned caesarean, so real life experiences much appreciated!

Thanks :-)

Ps: don’t have gestational diabetes.

21 Replies

My growth scans have been reasonably accurate, but I've just had an induction because of small birth weight, and I've had gestational diabetes so really wasn't expecting a small one, but my obstetrician thinks I just don't make big babies as they've both been little. At 36 weeks she gave me the option of induction between 37 and 39 weeks. I had my induction on Friday at 38+5 and she was 5lb 15oz, so she was as small as they'd thought.

You're so close to full term, you could maybe ask them about being induced early?

Good luck with it all xx

BristolLady profile image
BristolLady in reply to Seb9

Congratulations on the birth of your daughter!We’re not ruling anything out and will be interested to see how the growth is looking at 37 wks and will likely be keen to kick off the induction process around then…

Mine wasn’t a growth scan but with my now 7 year old I had a scan to check position of placenta at 36weeks. I was told she was over 6lb at that point and to expect her to be over 8lb at term. She was born 2 weeks post dates and was 7lb7.

I don’t know what’s changed in terms of accuracy in that time frame though as obviously advancements in tech can change quite quickly x

Thanks for sharing, helps confirm my thoughts that the estimations can be quite a bit out!


Mine was totally inaccurate, they scanned me loads and said it was going to be 8.7lb, two days later baby arrived and was only 7.2lb. Countless women I have spoken to said exactly the same, the measurements the sonographer takes are so subjective, and are often determined by the baby’s current position or even the sonographers experience. The thing is I didn’t feel that big and so I just knew the baby wasn’t going to be that big. How do you feel? Xx

BristolLady profile image
BristolLady in reply to Ladypii

Thanks! That’s the thing, am not feeling that massive, still very mobile, no aches or pains and the weight I’ve put on is average, if that (21 lbs) so just didn’t feel right that the measurement came back that high!

Time will tell… thanks for sharing :-)

Mine were completely off ... At 32w was told my baby is small ... 10th centile. Had 2-3 weekly scans, grew to 25th centile. Baby was born at 85th centile!

Gosh that’s quite a turnaround! Suggests perhaps not such a great sonographer?! Just shows it can work both ways, under and over estimations… thanks!

First time they certainly struggled with femoral measurement, but I had a few different ultrasonographers. Possibly cause I'm a fatty 🤣.

That was me a few month ago! We were always in the 97th percentile with body circumfence and I panicked a bit. Baby slowed down in the last 2 weeks but was still big. The later scans apparently are not very accurate generally though although they were for us. If baby keeps on looking big, they might suggest to induce not long after 40 weeks. Once thing that I would have wanted to know: big baby does not equal more damage, so don't be scared ;) Good luck!

Hi hun. My first baby was 9lb and my second just 1oz less. When I was having my third they told me he was going to be another big baby, when he was born he was only 6lb 11 oz! So they can get it wrong ☺ x

Sorry to not answer your question and ask you one, but do all pregnant Mum’s get birth weight estimates? I’m 25 weeks with my first

I got my first one on as part my scan report 20wks And then again at specific growth scans at 28 and 35 weeks.

Its calculated from head circumference, abdominal circumference and femur length.

Not sure if same across the country though, I’m in Bristol.

Maybe they can advise if you ask?

Joxxjo profile image
Joxxjo in reply to BRDouble86

No they only do growth scans if your midwife refers you due to bump measurements being too big/small or they thinks there's an issue with growth. If your pregnancy is straight forward there's no need to check

My NCT lead let us know that birth weight estimates can be up to 20% off. If you're feeling healthy and baby is fine then there is no reason to worry, intervention isn't always needed. My mother in law is 5ft2 and had 4 boys all over 8lb, 2 arrived over 9lb, so your body knows it's limits!

No it all depends on sonographer reading it right. My first growth scan was done at 32 weeks by an oldish woman who said my baby would be smaller than my first (7lb 11 born at 38 weeks) my next 2 scans they all said baby was going to be big (2 different people) and same as you at 36 weeks they said baby was almost 8lbs and if I went to term she would be huge. Needless to say first woman was correct and she came at 38.5 and weighed a teeny 6lb 8 😍 even with the extra 9 days to term she wouldn't of been as big as they estimated at 36 weeks.

BristolLady profile image
BristolLady in reply to Joxxjo

Gosh that’s quite the difference and much more than the 10% tolerance the consultant talked about. Should be having another growth scan in 10 days to see what that say, but most intrigued to see what he actually comes in at…. once we’ve decided how/when to deliver (indeed, if there’s a choice in the matter!)

Joxxjo profile image
Joxxjo in reply to BristolLady

Yeah it was way off 🤣 she was fairly long so I think they used that plus the size of me (I'm 5ft 2 and bump was all front plus 2 stone of lockdown weight from pre pregnancy made me look huge) and that swayed the decision. I wouldn't let the predicted size influence any decisions. And do let us all know the birth weight!

Mine was accurate. Delivered at 39 weeks exactly and weighed 8lb 11….I can only imagine he would have been ten pounds if I went over my due date. (No GD here either but they were convinced I had and tested me 3 times 🙈) x

BristolLady profile image
BristolLady in reply to Kyell2

Thanks KyellCan I ask if you had an induction at 39 wks - because of the baby’s size, or did it happen naturally? (or was it a planned c-section?)


Kyell2 profile image
Kyell2 in reply to BristolLady

Planned c section as the baby was beach and so chubby he couldn’t be turned 😂Ironically it was an ivf baby so I was having growth scans because they expected him to be small when it was the opposite, they did say his weight would have meant an induction at 39 weeks. But it’s completely your choice (I just wanted my baby as soon as it was safe to get him) x

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