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Right, sciencey types: I need hard info and I need it now!

(26 Posts)
phdlife Tue 10-Mar-09 08:44:34

Two questions:

1) sizing scans - how accurate are they?


2) VBACs - does baby's weight impact on chance of success? What else does?

Thank you

purplemyrtle Tue 10-Mar-09 09:00:48

1) Some say up to 25% out either way re birthweight, others 10% (underestimated ds1, always assumed they were overestimating!). However have been told they are much more accurate re head circumference, femur length etc, so better at size than weight I suppose.

2) Could do but impossible to predict as more to do with combo of size, position, what baby and body do during labour. Previous birth relevant, ie reasons why you had a section last time.

Hope that helps!

Does this make me a sciencey type?!

sarah293 Tue 10-Mar-09 09:09:30

Message withdrawn

YouKnowNothingoftheCrunch Tue 10-Mar-09 09:14:15

2. I would have thought that the mother's hips would have more of an effect than baby's weight - as the wider the gap the less pressure exerted on the scar IYSWIM

sarah293 Tue 10-Mar-09 09:16:16

Message withdrawn

Tangle Tue 10-Mar-09 09:43:32

1) multiple scans are thought to be more accurate than a one off.

2) How many CS's you've had, how straightforward they were, how much scar tissue you have. Position of baby, positions adopted during labour (VBAC women are often told they "have" to have continuous monitoring which means they "have" to lie on the bed for the duration - the former hasn't been demonstrated to improve outcomes and the latter is detrimental to most women's labour - oh, and no-one "has" to have either of them wink). Even the size of your hips isn't that much use, as its not a particularly good indicator of how much space there is for a baby to pass through - especially once your pelvis has loosened up in the late stages of pregnancy and your cocyx has moved out of the way (assuming you're not lying on it...).

Academicmum Tue 10-Mar-09 09:44:08

Hi - looked into this quite a lot when deciding whether to go for VBAC or repeat c-section. I think babies size only makes a difference if it is classed as exeptionally big (>4.5 kg). Other things which can affect are 1. mothers age (if over 35); 2. smoker/non-smoker; 3. previous successful vaginal birth; 4. reason for previous section; 5. direction of scar (vertical scars are I think almost never recommended for VBAC); 6. mothers weight; 7. induction of labour; 8. number of previous sections.

theyoungvisiter Tue 10-Mar-09 09:59:58

sizing scan - individual scans aren't very accurate IME though it depends on the operater. I had several scans for DS1 and in one or two scans the measurements were waaaaay out (one scan put his head on the 3rd centile and another put his abdomen at 90th centile - in all other scans he was on the 50th for both, which was also where he was when he was born).

Repeat scans are meant to be much, much more accurate.

Not sure about vbac - I think perhaps it depends whether size was a factor in the previous decision to perform a section?

phdlife Tue 10-Mar-09 11:22:37

thanks for this.

it's a toughie -

near as I can tell (after an hour googling vbac info), babies' size does impact on success of VBAC, especially if you haven't had previous vaginal delivery. This study found that for 2nd babies over 4000g the VBAC success rate starts at 68% and drops to around 38%; also that if baby weighs over 4000g chance of uterine rupture was double that of the rest of the group. Another study I saw found that morbidity rates in unsuccessful VBACs were twice those of repeat elective c/s.

So given that Ds was 3940, we don't really know why he didn't come out, and #2s tend to be bigger, I guess it would be sensible to have the scan...

phdlife Tue 10-Mar-09 11:59:44

ok here's another one - who can find some actual factual statistics on whether doing ante-natal exercises helps or not?

MustHaveaVeryShortMemory Tue 10-Mar-09 13:18:39

Helps what? Having a VBAC or continence or managing labour?

childrenchildreneverywhere Tue 10-Mar-09 18:48:37

Lots of info re. growth scans and VBACs with large babies here:


mumoftoby Tue 10-Mar-09 20:39:33

I had some scans at about 34 and 36 weeks with DS2 (due to a lack of foetal movement). There was a little concern as they estimated he was on the 50th centile, which is a bit smaller than DS2 who weighed 8lbs 12 ozs at birth. I actually asked the consultant how accurate the scans are she said to within 10%.
Well 'smaller' DS2 was born weighing 10lbs 14ozs! So I personally really don't think the scans that I had were very accurate.
purplemyrtle unfortunately it was as inaccurate for size as weight. He was extremely long at birth and even now is above the 98th centile for length and only the 75th for weight.

purplemyrtle Wed 11-Mar-09 10:00:09

I've had repeat scans in both pregnancies, both times predicting big babies, which both babies were. I don't think they can be spot on but I think they can be useful to give an idea, especially if as in our case there were complications the first time which may have been due to size. I wonder if it also depends on machines used and particular sonographers? I decided against VBAC partly based on scans etc, and in retrospect feel we did the right thing, might not be the right thing for everyone though, and people do have successful VBACs in similar circumstances.

LucyEllensmummy Wed 11-Mar-09 10:17:37

I don't know much about the sciencey bit but i do know that I was sent for a scan at 39 weeks because i was large for dates - scan predicted 7pound - one week later NINE POUND NINE!!!!

LucyEllensmummy Wed 11-Mar-09 10:18:07

sorry but what is VBAC? often wondered

Tangle Wed 11-Mar-09 13:06:43

VBAC = Vaginal Birth After Caesarian

phdlife Thu 12-Mar-09 11:07:44

musthave I wondered if doing ante-natal exercises helps with a vbac (or any vaginal birth for that matter). don't suppose it's the kind of research that can be done terribly easily though.

phdlife Thu 12-Mar-09 11:11:55

and I do wonder about sizing scans, because if the chance of vbac success drops from 65% to 38% between 4000g and 4500g, I'd really want to know if we were talking a 4500g baby - that'd probably be enough to change my mind about trying for a vbac.

Lulumama Thu 12-Mar-09 11:13:30

1) i believe the margin of error is around 10 - 20 % , which is quite a difference.

2) some doctors/MWs say a slightly heavier baby is easier to birth than a lighter one, as the baby's weight helps it out.

chances of success really rely on a spontaneous start to labour, a well positioned baby , an active / upright/ mobile labour, and i think to some extent, being positive about your chances.

DS was 7 lb 13 with a head circ of 35 cm, and was a c.s for failure to progress after induction at 39 + 1 due to complications, the entire labour was immobile and i had an epidural v v early on.

DD was 7 lb 8, and i had a growth scan as i was told she would be mahooosive, certainly bigger than DS, and her head circ was 32 cm. she was VBAC, and OP and born OP. no epidural and lots of time on my feet.

Poledra Thu 12-Mar-09 11:15:23

Don't know about sizing scans, but my VBAC babies were 8lb 14 (4.05kg, I think) and 8lb 11. With mahoosive heads (both above the 95th centile).

phdlife Fri 13-Mar-09 11:23:20

agreed, lulumama - if we are talking a difference of a couple of hundred grams it is pretty significant. And I've seen research on the importance of a spontaneous start to labour. But I haven't found any hard research that supports the view that active/upright/mobile labours help - in fact iirc said there wasn't any. And that is the kind of info I am after atm, to help make this decision. It is hard to "be positive" about my chances when statistically they are against me, and being unsuccessful would incur a significant risk.

Lulumama Fri 13-Mar-09 16:30:32

an active /upright/mobile labour allows you to work with gravity, and for your baby to get right down onto your cervix. being flat or sat dwon can stop your pelvis opening up to its maximum. so even without research, the mechanics of labour and the pyhsiology would indicate that working with rather than against gravity would be good.

or to put it another way, being allowed to work with your body and tune in to it, rather than being stuck on a bed, with lots of meds that make moving impossible or very restricted would be a lot better?

vbacqueen1 Fri 13-Mar-09 18:56:40

First baby 4lbs 13oz - caesarean, failure to progress.
Second baby 7lbs 1oz - elective CS.
Third baby 8lbs - elective CS.
Fourth baby 9lbs 10oz VBAC. Scan couple of weeks previously was spot on (as indeed were all of them for previous pregnancies too although I know this isn't always the case for everyone)

I'm a big believer in a lot of labour going on in your head - believe that you can do it, be active and upright as lulu says, and the rest usually follows, no matter what the size of the baby. They are, after all, designed to be pretty squashy wink

Bex22 Sat 14-Mar-09 21:58:08

I was induced at 38plus weeks cos sizing scans and size of my tum all indicated that baby would be absolutely enormous- over 100th percentile or whatever the top line is called. DS had been 8 and a half pounds and 10 days overdue so was also induced there. I was absolutely freaked out for the last couple of weeks because I had been convinced that I was going to have a 12 pounder and would need an emergency caesarian or forceps. It completely spoilt that last couple of weeks of pregnancy plus I was made to have a very unpleasant diabetes test too which involved drinking copious amounts of sugared water-- shock.
Anyway, to cut a long story short, was induced, it worked very well and had a very short labour (3 plus hours) but was shocked that baby was a very normal 7lbs 14oz!! Midwife who delivered her was the only one who actually looked at my tum (in between my screams of pain) and said that she thought she was just all at the front and not such a big baby after all- the sole voice of reason after talking to numerous doctors, consultants and midwives!!
It was obviously less painful giving birth to a smaller baby, but I did feel a bit misled afterwards into having been led towards induction. Admittedly, I was keen on this at the time, but who wouldn't be when faced with birthing the world's biggest baby!!?! Moral of the tale- sizing scans- baloney!!

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