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How accurate are date changes at 12 wk scan

(15 Posts)
EffinIneffable Fri 23-Jan-15 17:45:28

I went for my 12 week scan at what I thought was 12+1 and they changed it to 12+5 based on measurements (CRL of 63mm).

I'd had previous reassurance scans due to previous mcs at 7 weeks and 10 weeks that were spot on my dates. I'd also been tracking ov with CBFM and we actually only managed to dtd once that cycle blush so I do have a pretty good idea of when we conceived.

I know that tracking and early scans are not 100% reliable for dates, but is it not possible the baby's just had a growth spurt and is a bit larger than average at this point? The only reason I'm concerned is in case they get a bit eager to induce when the time comes based on dates that aren't really that accurate.

Showy Fri 23-Jan-15 17:49:44

They're accurate to about 5 days either way. Regardless, nearer the time you can say no to induction. It doesn't matter in the end what a scan tells you. They come when they're ready. If you don't want induction and they start pressing for it, they can offer monitoring and make decisions based on what's actually happening and not the date on the calendar.

EffinIneffable Sat 24-Jan-15 09:15:43

Thanks showy - 5 days either way is quite a lot of leeway really. I had some friends who were put under a lot of pressure to induce when they were late with their first, based on population stats rather than indications on what was happening with their individual baby. I think I will chat to the sonographer about it at my 20 week scan.

Flossieflower Sat 24-Jan-15 09:19:11

My friend had her dates changed by a week at the 12 week scan- was an IVF pregnancy so they know when conception was to the hour! She then had a c section at "39" weeks which was actually 38 weeks- not ideal.

Nottalotta Sat 24-Jan-15 15:07:20

Mine has changed by 12 days! GP said 13th Aug, midwife initially said 8th then revised to 5th based on my short cycles. 12wk scan yesterday and now changed to 1st Aug. I do have short cycles and I do ov early.....

PTAblues Sat 24-Jan-15 15:24:51

Ds they put at a week further on and I knew exactly when we did the deed. You're right in that it only causes problems when you get to full term- or what they think is full term. Noone ever listened to me and insisted on going by the dating scan and this meant I had to go through a really uncomfortable sweep when I knew I wasn't overdue. They had also booked me in for an induction but my waters broke before that (2 days after my own due date).
I often wonder if this blind disregard from information from the women themselves causes more problems and wastes more money in carrying out inductions that are unneccessary.

EffinIneffable Sat 24-Jan-15 15:37:09

I can't help but think they must base their dating on the average sizes of babies at different dates - but by definition some babies will be larger or smaller than the average on any given date.

I've also had arguments about my dates when I've had previous mc's and they've told me I just got my dates wrong,, everything's OK I'm just not so far on as I thought, when I've been obsessively tracking everything!

Lots of women won't know precisely when they conceived (and even if you know when you dtd it's not 100% to the day) but a bit more trust of our knowledge of our bodies would go a long way.

seaoflove Sat 24-Jan-15 15:41:24

I went for my scan at 12+4 and got changed to 13+5! I know exactly when I ovulated, and only had sex once that month, but no one is interested. As it happens, I'm perfectly happy to have the pregnancy cut short by 8 days, but it IS annoying when they tell you that your dates must be wrong and it can't be a case of the baby measuring large.

MrsMogginsMinge Sat 24-Jan-15 16:45:16

I've had a similar change. I was using OPKs that month so I know I ovulated on day 17 (as normal for me) and DH had an overseas work trip so I know when the sperm was delivered. So I expected to be measuring a few days behind, but measured bang on at 9 weeks. Apparently if you have an early implanter that can make up few days' difference in growth (and would fit in with me getting an undeniable BFP 10 days after ovulation). So I was happy to go along with that. Then at my dating scan I was moved another 4 days ahead - I even made the sonographer measure it twice. I was also told there's a 5 day accuracy range. Anyway, I'm 36 weeks now on their dates (35 on mine) so I can't tell you how the story ends. I come from a family of slow baby cookers (and big babies) so I'm preparing myself for an induction standoff next month... Am resigned to the fact that due dates are an inexact science and will have to see how 'imminent' I feel at the time.

SunbathingCat Sat 24-Jan-15 16:56:47

Message withdrawn at poster's request.

Maxis1 Sat 24-Jan-15 17:28:34

The joys of "wannabe half-gods in white" thinking they know better. Between the different scans I have been told 8th (at 13 weeks), 11th (at 22 weeks) and 4th of February (at 26 weeks) and calculated 9th myself. If I count in family history of slightly early babies I am banking on 7th for now.

I sometimes wonder what they would do if they book you for an induction and you just dont show up that day or week if you know the dates they have calculated are wrong ;)

Showy Sun 25-Jan-15 19:00:08

Maxis, they can't book you in for an induction if you don't want one. You just say no. You don't even need to not turn up and waste time/resources.

Babies develop pretty uniformly up to 12 weeks which is why your dating scan is around then. The baby is big enough to measure but small enough that genetic propensity to be bigger/smaller than average hasn't really kicked in yet.

There are lots of things which can make dates change once you're scanned. Obviously, they can only be accurate to around 5 days either way so you have that margin for error. Mostly because babies don't cooperate . So if you're trying to measure crown to rump and they're curled up in a ball or flipping around or generally being awkward, you have to accept that you might be out slightly.

And most women don't know when they ovulate. OPKs don't show when you ovulate so can't be relied on. Sperm can live for over 5 days inside a fertile woman so even knowing when you had sex isn't a guarantee. But say you do know exactly when you ovulate, the time taken for the egg to travel down the fallopian tube and implant can vary by many days.

If you take all of these different confusing factors for predicting dates, it's not surprising there's some moving about of dates. It's bloody amazing they're as accurate as they are. Maxis, for example, you calculated that you were due on the 9th, scan said 8th. And it only moved a few days at the later scan when dating is much, much less accurate. That on its own is remarkable isn't it? That they can see so much and so accurately? I tend to think it's nothing to do with doctors being Gods and arrogant. They have to arrange care. Women do need monitoring and at specific times if they want things like certain tests or have specific needs and scans are absolutely the best way we have right now to do that. The alternative is that you base care on what a patient says and while some women may know categorically, there are myriad reasons why a woman might seize the opportunity to be less than truthful about the time of conception. I acknowledge completely though that no medical professional has the right to tell a woman she is wrong, to undermine her or to force medical care on them that does not fit with what they know to be true.

I do see the problem with them pushing induction too soon in particular. Or sweeps. But you can say no. A due date is arbitrary anyway. I wish they'd move to acknowledging that a normal pregnancy is 37-42 weeks and there's no 'overdue' or 'early' within that. Only normal. It'd save a lot of piddling about and pressurising heavily pregnant woman or making them feel like a watched pot.

PTAblues Wed 28-Jan-15 09:49:03

I was told the whole sperm can live inside you thing by my midwife when I questioned it and I understood that. Thing is I kept trying to explain to her that as they had dated me a week early then if the sperm had been hanging around for days after we dtd then that was making the scans date of conception more wrong. She couldn't work that one at all and kept repeating the sperm thing to me.

Maxis1 Wed 28-Jan-15 14:33:06

at the last checkup on Monday my consultant was off and I had to deal with a replacement one. She was all about well we will talk to you about eviction methods when you are between 10 and 14 days overdue. Shou have taken a picture of her face when I told her that that doesnt work for me and I will not let them let me go over by that much.
due back with my GP next week at 39+5 and the consultant at 40+2 but I still hope I can convince the little one to come out a litte sooner than that ;)

naty1 Wed 28-Jan-15 19:25:31

My ivf dc1 was dated about 3 days early but apeated a few days earlier than that. Ivf babies are often early.
Dc2 is being dated 4 days early so im expecting them at least a week early.
Im worried about a big baby and would not want to go 14 days overdue so would take the induction. I would be more concerned with being dated late and not being induced when i need to be, already having a difficult birth with a 7lb 10 baby 4 days early.
I think being stuck in early labour for days/ weeks would be much worse than induction as you wouldnt know when it would stop and even getting from 2-4cm can be really painful depending on the position of the baby etc.
Though i honestly dont think they should have the right to change dates with ivf.
When a few weeks after birth DDs head was measured it was 91st percentile (dp has long back too) and despite now only being 50th centile for height her head is still large. I think that probably contributes to measuring large on scans.

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