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Dating scan date change

This is frivolous really, but anyway I'm curious...

I was sure when we'd conceived due to ov date, DTD days and when symptoms started. We had a private scan at 7+1 (by my dates) and they dated me at that. The lady scanning me said 'your dates are spot on, don't let them mess you around and change it'.

I had another scan due to bleeding and working myself in to a tizz at 11+1, and the private scan put me at that also. When I had a dating scan at 11+6 at the NHS it put me a 12+2, so 3 days on. I'm assuming mine will be a big baby. I had a baby with a 6ft man (I am not particularly tall, my bio-dad was 6ft 4) and had a 9lb 5 baby in '96, and he's now over 6ft 6.

My husband is 6ft 3 and very broad, his sister is 6ft, so I am thinking him+me could equal a bigger baby than before. Questions:

1. How could a scan 5 days apart date me 3 days differently?

2. Assuming my original dates are right does genetics play a role at such a very early stage? Is the 12+2 (vs 11+6) because baby is bigger than average? Or do the variations start later in development?

Thanks for shedding any light!



  • Id say variations start later. Id not worry. My h is 6ft4 and our son was 5lb 6

  • Dates do change hugely, because not every embryo / foetus grows on a uniform curve. It is normal to have a growth spurt (so measure further ahead) and then plateau (so potentially measure further behind) and everything in between.

    I went for a scan at 8+1 (by my dates) and they dated me 8+4. I KNEW this was wrong and tried to argue it, but the whole point of my early scan was to date me 'accurately' so I could be booked in for a CVS at 11+3. I duly got booked in for 11+3 by the 1st scan dates (11+0 by my dates) and when I arrived and was scanned, they couldn't do it because I wasn't far enough along (that scan dated me as 10+5). They re-booked my CVS for 6 days later, when they thought I'd be 11+4 (by the 2nd scan) but I knew I'd be 11+6 by my dates. Fortuntately they were able to do it, although the 3rd scan dated me at 11+6 - finally matching my own dates!

    I therefore have 2 EDDs recorded in my notes - neither of which is correct, as they recorded the 1st and 2nd scan dates, and are 5 days apart!

  • I had an early scan at 8 weeks, which put me back a week. Then at my 12 week scan, they put me forward by 4 days.

  • The other thing about scans is that they put a dot on the screen where they think the baby starts and finishes and measure between them, they normally do it 3 or 4 times and average the results because it's not supremely accurate! J got moved ahead 5 days at my 12 week scan, so I thought he might be big - 6lb3, so not so much!

  • I don't know why you were dated so differently in such a short space of time, but i guess babies are developing quickly in the early weeks and that's why.

    By my dates I thought i was due 20th of May this year as i knew when i'd OV'd etc.  At my 12 week scan they gave me an EDD of 16th May.  Baby arrived on the 29th of May weighing in at 10lb4oz.  by the hospitals dates i was 12 days late when i was induce.  by my dates i was only 9 days late.  I read an article about how human pregnancies can swing 5 weeks either side of your expected due date, and there was a question of whether induction at 12 days over your EDD was actually a good idea or not.

  • Different people are also bound to get slightly different results on different days surely. Like said, it's still estimation and human error. I wouldn't say that genetic differences would affect at 12 weeks, up until this point they're meant to be pretty much the same. With J I had a couple of early scans and my first moved me back a week by my dates (at 7 weeks they said I was 6 weeks), then at 8 weeks they moved me forward by 4 days and by 13 weeks I'd been moved forward another 4 days.

  • I knew my dates were wrong as I knew when we conceived. They put me back three days at my scan (good thing too as my original due date was my birthday!). He still arrived 11 days early so I wouldn't think about it too much.

    My H is 6ft and N was 7lb 8.

  • Thanks ladies! I'm not worrying, I just like to (where possible) understand stuff :)

    I thought it was too early to be a genuine variation in size as against average, but thought I'd ask. I went 17 days over with Harry, but my original dates that would have been 13 days. I can't help but think they 17 (which is extreme) was due to size differences not genuine timing.

  • I always think the 12 week measuring is odd as it fits everyone to average I knew when we conceived p but was put frwards a week at 12 weeks. However between 20 and 34 weeks I had fortnightly sans as she measured small. SHe was 6lb1oz when born so not big at all. so it's a guessing game and can go either way I reckon!!

  • When you think about the teeny tiny little baby they were measuring at 7 weeks, we're talking tiny mm, any tiny discrepancies can change the date by a way. The bigger the baby gets the easier and so more reliable. My initial date for E was 23rd December.  At 10 weeks I was moved to 28th and at 12+5 to 4th January. Also you may know when you dtd but pinpointing ovulating to the exact hour is difficult and you can't say when that sperm met the egg (remember it could be up to 7 days later) and you also can't say when the fertilised egg implanted either. I personally wouldn't worry.  I didn't x

  • I was moved both times. The first time I was 100% certain of my dates as they were backed up by cd21 blood tests. I was moved forward 6 days, my concern was beig induced earlier than 40+14 by my dates. As it turned out he was born on the EDD from the scan. He was 9lb, so fairly big.

    With M, they only moved me forward 4 days, and she was born at 40+5 by that date - or 40+1 by my dates at a rather diddy 7lb5oz.

    They base the dates on being 50th centile, so bigger babies will be dated slightly ahead, but from what I understand it's at most a week either side. It's worth mentioning it to your mw in case you do go over and want to avoid induction.

  • BG - you're right about the induction thing. I was induced with my son and it was fine but it's not my preference as shooting for a home birth and the days (at a late stage) could come in to play. I'll bear it in mind for now and see how we go, thanks :)

  • LM - that's true if they are moving you back, but you can't conceive before you ovulate, which is what they were doing with me, and Inhad a CD21 blood test which confirmed I couldn't have ovulated when they said I had.

  • I did know when we conceived, to the hour as we were ivf and still got moved forward three days.

    I think even if measurements are completely spot on, or ov / conception dates are, the dd is still such guess work as our bodies all labour differently. Though is it our body or the baby that starts labour?! It's very interesting.

  • I know I can't be 12+5 (per Monday's scan) due to ovulation, but I'm listed as that now. I am not counting on it AT ALL for EDD and have warned hubby to have 2 weeks over in his head to manage expectations, and based on last time. So when someone asked when I would pack up work the date I gave was later than my EDD (in an ideal world, obviously) because I don't have mat leave/holiday days to toss away :)

  • Unfortunately you can't start mat leave any later than EDD or when baby is born, whichever happens first.

  • True, but I'm banking on them being flexible before and after. There's little in monetary perks there but they do try really hard to accommodate personal situations from what I've seen so far. Hopefully it will be irrelevant and I'll go in to a natural labour a week early ha ha :)

  • I'm not sure how flexible they can be on that though, to claim SMP they need to produce your matb1 form which will have your EDD on as entered by your mw.

  • But yes, hopefully it'll all be irrelevant when you go into labour at 39 weeks.

  • BG - I could start mat leave but work extra to boost my lieu/hol balance to use later in year. I'm hoping to be able to go back before I officially return, part-time, to save lieu days for later again. A phased return that means I won't lose cash IYSWIM. Things are going to change a lot at work between now and then though, so I might just have to swallow doing it by the book :)

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