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Calculating due dates when MW and scan says otherwise anyone had this happen to them?

(10 Posts)
broodymom Thu 23-Oct-08 15:02:33

Right i'll try and keep it simple hmm !!

We have been ttc for over a year charting cycles etc and marking each day in diary. So the first day of my last cycle was 22nd june thats a definate. I tested and got bfp on day 52 (was scared to test!) so made appt with gp and had pg confirmed. At my booking appt mw put me at 11+3 weeks according to lmp.

so sent for scan the following week and baby measured 8+3! so i said ok

went for my 12 week scan and they couldnt get a proper measurement but what they could get suggested 12 weeks min gestation.

I'm now 14+5 according to scan dates, but i would be 17+4 according to lmp.

How does this work out! they keep saying i must have my lmp wrong but i know i deff dont. So is my baby small or are they/me wrong?

Am i going to have an early baby or a really overdue one? anyone help please x

CookieMonster2 Thu 23-Oct-08 15:13:58

It does sound like there is quite a difference, but if you are really sure of your dates I think you need to make that clear to the midwife and get the due date changed.

My 12 week scan showed I was a week further on than what I had said. I was concerned that when the due date arrived they would be saying I was overdue when I know I wasn't. I know my dates are right, I know what day I ovulated and I told the consultant this, but he didn't seem bothered. My midwfie forgot to update my due date anyway so I didn't remind her.

At the 12 week scan (or at mine, at least), they measure the baby, plot this on the 50th centile line, and then calculate the due date from that. So unless you have a completely average baby your dates will be more accurate.

flynn1984 Thu 23-Oct-08 15:19:41


When I went to see my midwife for my first appointment I told her the date of my last period was 24th august so she put me at 7 weeks and I'd be due 1st June. However when I went for my early scan there was only an empty sac which measured at 4-5 weeks and that caused worry as thought I was miscarrying.
Went for another scan yesterday and told me I am now around 6 weeks as yolk sac has now developed.
I had very irregular cycles which could be the reason for this mix up - so in my case I trust the scan dates rather than the mw due around 17th June.

SmileLikeYouMeanIt Thu 23-Oct-08 15:27:48

Similar thing happened to me when expecting DD
My EDD according to LMP was 1st July, after my 12 week scan it was changed to 20th July.
I knew this wasn't possible as I'd have conceived 2 days before my positive test!!
DD was then born at 36+4!
I was adament my dates were right but she was still officially classed as prem which I did find very irritating!

Tee2072 Thu 23-Oct-08 15:29:38

It is possible you ovulated later than you thought you did, but still managed to catch. That could cause the discrepancy, even though you know the LMP date.

sorkycake Thu 23-Oct-08 15:33:07

Tee is right it's highly likely that you didn't ovulate until very late in your cycle. Stress, illness can delay it. Testing positive on cd52 could in theory mean you didn't ovulate until somewhere between cd35-40. This would make you 2-3 weeks later in terms of dates by LMP.

Go with the scan date.

JustKeepSwimming Thu 23-Oct-08 15:34:36

AFAIK the MW using the LMP circle thingie assumes that all women have a 28 day cycle.
so LMP is day 1
ov is day 14

in fact the length of time between LMP and OV can vary massively in different women (& in the same woman in different cycles, i know this from experience!) so this is a very misleading way of calculating due dates.

the scans, esp early on measure very precisely certain things (sac size, embryo size, etc.) which while they can vary, vary a lots less than cycles can.
so go with the scan dates.
And make sure the MW changes your notes to agree or you may get pushed into induction early or be classed as prem when you're not - see Smiles post.

congrats anyway

CookieMonster2 Thu 23-Oct-08 16:12:24

OK I'm out voted grin But I still maintain that in my case I am right because I know the ovulation date.

JustKeepSwimming Thu 23-Oct-08 17:15:18

Ah but Cookie - they can be wrong from scan dates too - sure there was a thread a while back about someone who was given dates that meant she must have been unfaithful to her DH!

Depends on OPs cycles really - if she normally has 28-ish day cycles then there is a discrepancy...

Before ttcing ds2 was watching my cycles (occasionally temping) and i was ov'ing on day 20/21/22, to get pg for ds2 i must have ov'ed on day 10!
caught out? just a bit! grin

broodymom Thu 23-Oct-08 21:27:36

Thanks all i feel a bit better about it now. My cycle is not 28 days it is on average 35 days once it was 39 so i guess i will go with the scan and speak to mw on tues. Its all so confusing, thanks again all xx

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