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Is due date from 12 week scan more accurate than, say, 6 week scan?

(10 Posts)
Hadeda Wed 01-Jul-09 14:26:44

I'm currently 40 + 3 (perhaps) and am just trying to think things through in case we get to 42 weeks.

We had an early scan at 6 weeks which gave my due date as 3 July. The 12 week scan gave us a due date of 29 June. The hospital/midwives have always taken the due date as 29 June.
With my DD we also had a 6 week and then a 12 week scan. 6 week gave a due date of 4 Feb, 12 week gave a date of 1 Feb. I went into labour on 4 Feb, so I feel the early scan was more accurate.

I have read that early scans can be more accurate. But more accurate than what - LMP? Or later scans? And why is the 12 week scan taken as the due date rather than the 6 week scan? Is it because everyone has a 12 week scan or is it actually more accurate in some way?

I'm booked to have a HB and you can't have one past 42 weeks. I probably won't argue about going into hospital if we go past 29 June + 14. But I don't want to be induced until 3 July + 14 unless there is some sort of problem. Does that sound reasonable?

I really hope the baby comes soon so I can avoid all this!

jaype Wed 01-Jul-09 14:46:41

I'd go for your LMP, adjusted for your usual cycle (if you had one) - I used to have a 37 day cycle and surprise - my 1st dc was 11 days late. I was fending off midwives armed with induction drugs right until the very last!

audreyraines Wed 01-Jul-09 14:46:47

hope you go into labour tonight hadeda!

if not, ask your MW about expectant management rather than being induced. That means they will monitor yours and baby's health and only induce if there is an indication that it is not safe to wait for spontaneous labour.

i would also bring up the issue of dates. do you have a LMP date and what does that say? if you don't, you could probably argue the 6 week date and avoid induction until 3 July +14. i think that sounds completely reasonable. and anyway, they can't force you to have an induction at a particular time.

hope all this will become a moot issue though shortly!

audreyraines Wed 01-Jul-09 14:55:56

have a look at this, it's an excellent resource.

CMOTdibbler Wed 01-Jul-09 14:57:52

The 12 week is more accurate as the dating is done by the size of the baby, and as at 6 weeks they are tiny, the error on the ultrasound machine (given as +/- 2mm by the manufacturers) plus that of the operator means that they can be well out

Tangle Wed 01-Jul-09 16:29:26

See I heard that the 6 week should be more accurate as they can use the development of the major structures to pin point gestation fairly accurately. By the time you get to about 12 weeks all the obvious features are in place and all that changes is size and you're then comparing measurements (which aren't 100% accurate to start with) with average growth curves.

(and I know this is as semantics issue, but its one of those things... blush - you can have a HB past 42 weeks and although your HCPs may recommend against it they have a duty of care to support you in your choice. They can't force you into hospital against your will without proving you're mentally incompetent and getting a court injunction!)

Hadeda Wed 01-Jul-09 19:29:12

Thanks for the messages, and the link. I plan to raise the issue of dates with the MW, or with any consultant I land up having to see. I fell pregnant with this baby before my periods came back - was bf and came off the pill so periods could resume but the baby got there first... wink So I have no LMP to go by, just the scans.

I really hope we don't get there, but wanted to have thought about it at least.

audreyraines Wed 01-Jul-09 21:22:09

i think it's wise to at least think through your strategy. as has been said, the hospital has a duty to send someone to your home when you're in labour and you have no obligation to go into hospital. you can only take their advice, consider it and then make your own decision as to whether you want induction, or to allow things to progress at home as you wish. good luck!

traceybath Wed 01-Jul-09 21:25:11

12 week is most accurate according to my midwife and consultant and is what my dates are based on.

I also had no lmp as bf. Therefore had a couple of early scans and they varied quite a lot in terms of dates.

Good luck!

WesternBelle Wed 01-Jul-09 21:38:13

My 1st scan at 8+1 was more accurate than the 12 week scan.

8+1 scan dated me 4 days later than my own dates.

12 week scan dated me 9 days later than my own dates (and 5 days later than 8+1 scan, obviously).

Baby arrived 9 days "early" (ie my due date)smile Twas a first baby too, often known as being more late than early.

I never did trust those scan dates entirely...

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