My feed

to access all these features

Get updates on how your baby develops, your body changes, and what you can expect during each week of your pregnancy by signing up to the Mumsnet Pregnancy Newsletters.


Accuracy of scan dates

13 replies

Spindelina · 21/01/2017 20:32

I've had several scans this pregnancy. By LMP, my EDD would be 19th July. I've been measuring as follows (dates of scan by LMP):
6 weeks: EDD = 21st
7 weeks: EDD = 22nd
9 weeks: EDD = 21st
13 weeks: EDD = 16th (but they only got one measurement, and it wasn't great, hence today's scan)
14.5 weeks (today): EDD = 11th

If I'm really 15+3 (or whatever it is that makes my EDD the 11th), then I had 6 days between start of LMP and conception. That doesn't sound right - my cycles are usually 28-30 days.

Trouble is, my official EDD is now the 11th. DD was born well past her due date, as was I, as was my brother (all 12-15 days over). I don't want to be induced unnecessarily based on one dodgy measurement.

So, questions: are there any pathologies or other reasons that the head circumference should be so out of line with earlier CRL measurements? How reliable is head circumference compared to the other methods?

And is it worth discussing with my midwife sooner rather than later, or should I just wait for 42 (official) weeks and then request daily monitoring?

OP posts:
Spindelina · 22/01/2017 08:14


OP posts:
MouseLove · 23/01/2017 08:39

The early scans are a bit hit and miss and you might get your date moved again at 20 weeks. I wouldn't worry too much until after that scan. Maybe raise your concerns with your midwife and double check everything. If you're not happy you could possibly go for a private scan?

I know exactly how you feel though. I'm 8 weeks today by LMP but baby measured 5-6 weeks on Saturday when I had to have an early scan. Everything looked fab on scan but it does leave you scratching your head.

ArialAnna · 23/01/2017 08:57

My EDD also got pushed forward (by three days) which I found a bit puzzling as very very rarely is my cycle less than 28 days.

Pushing it forward by 8 days does seem quite a lot, so think it's best to raise your concerns with your midwife. As it's not an exact science, she might be willing to change it to a date halfway between the two, so you'll be at less risk of being unnecessarily induced.

savagehk · 23/01/2017 09:49

Personally, I knew my exact date of conception with my first baby (I'd just been charting temperature and knew exactly when I'd ovulated and also when we'd had sex, so I was certain - if anything and it's the 'sperm live 5 days thing' conception would have been later rather than earlier).

Scan date dated me at 13+5 when it should have been 13. Midwives 'agreed' to use my date but then once I hit 42 weeks by scan date I was 'too overdue' for home birth. If I could do it again I'd stand my ground, you can refuse any interventions you wish to including induction. I booked in for an induction at 42 weeks by my dates (rather than theirs) but labour started at 41+6 by my dates (but 42+4 by theirs). I had a placental scan at 42 weeks and then went for expectant monitoring. I would raise it at every midwife appointment you have to ensure everyone's on the same page and would probably push for a changed EDD if you are able.

This time, my scan was late (I was unsure on my dates), so when they did the scan they decided to date me by head circumference as you have had, as baby was too long for fetal length. Working backwards it gives me an unworkable LMP period date, so I expect I've been moved 5 days earlier again. My firstborn had a head that was 99.8th percentile at birth, so dating me by head circumference is probably a bit optimistic anyway if I produce babies with large heads!

I mentioned this in passing to the consultant who I saw post scan and he said that later scans (eg past 13+5) are less accurate generally so I'd hope in your case the 3 earlier scans would trump the last 2 peculiar ones. I'd be surprised if they move the date at 20 weeks again. I was told last pregnancy that if you had an 'early scan' (before 13+5) that is what determines your 'due date'.

user1485166754 · 23/01/2017 10:32

i have had 2 scans and 3 different dates all differing within a 10 day window. ill probably get another date at my next scan!

arbrighton · 23/01/2017 10:32

I've been moved 5 days earlier but given I expect they have quite a lot of data to give them their measurement = due date calculations, I'm choosing to believe them

I was scanned at 12+3 on LMP, but moved to 13+1

Blondeshavemorefun · 23/01/2017 10:46

I got shifted a few days then forward but as ivf I knew excalty when conception was and when egg back in so always went back ivf clinic dates

Spindelina · 23/01/2017 10:55

Thanks all.

savage, that all chimes with my experience. I shall make sure that everyone is aware that I don't agree, and try to get date changed.

I've been looking at charts btw. Assuming this fetus hasn't had a masssive growth spurt (and that the discrepancy is because of the measurement uncertainty), then...

  • you need to measure an 11mm embryo as 4mm to get a date of 6 weeks from a 7+1 embryo, so error of over 50%
  • you need to measure a 17mm embryo as 9mm to get 6+5 from 8; error of over 50%
  • 8+5 from 10 week fetus is 22mm from 34mm; error 36%.

  • to get 15+4 from a fetus that is actually 14+3, you need to get a circumference of 117cm when the true value is more like 105mm. That's an error of only 11%.
OP posts:
FATEdestiny · 23/01/2017 11:00

I have tracked my ovulation and menstrual cycle for 15 years - for both contraception and conception. I know exactly when each of my four children were conceived and have an exceptionally regular cycle.

I have also carried all for of my children 6-8 days overdue from my date. The time I went overdue according to EDD varied from 6d to 16d, but that was due to hospital moving my EDD at scan. My dates are consistant.

So when my EDD for DC3 was moved 8 days earlier, I knew it was wrong. When they wanted to induce me at 40+12 I refused. Not because I was blase about the (very real) risks of going past 42 weeks. But because I knew I was in fact not even at due date yet, and baby would be a week late any way.

It was very stressful to refuse induction. You need to be very certain of yourself because i was put under a lot of pressure, with daily monitoring at the hospital (during which several people would spend 20 minutes lecturing me about the risks).

True to form, DC3 was born 40+16 from EDD, which was 40+8 from my date and entirely predicted by me as when baby would come from the beginning.

Spindelina · 23/01/2017 11:33

FATE, I suspect that's where I'm headed. On the plus side, it's a pleasant 30 minute walk (60 minute waddle?) to the hospital for my daily stroll...

I was very clear with DD that I didn't want to be induced until 40+14. She was born uneventfully at 40+12 after two S&Ss and ARM which, armed now with more knowledge of the possible consequences, I think I would decline (at least until 40+14).

OP posts:
Rubberubberduckduck · 23/01/2017 13:40

This reply has been deleted

Message withdrawn at poster's request.

Spindelina · 23/01/2017 15:01

Rubber, I have no real idea when I ovulated, and can't pin a DTD date. So I don't really have anything to stick to my guns about - just a niggling feeling that ovulating on CD6 is pretty unlikely. Ovulating on CD14-17 (as per early scans) seems more likely. Ho hum.

OP posts:
vfoster · 23/01/2017 16:11

I went in at 12+2 by my dates and they dated me at 13+1. Looking at my timings that's pretty much impossible as my cycle was pretty short and I was using ovulation sticks.
My first baby was 10lb 10 (very long!)so think this baby is just another long baby! I've refused induction anyway so hope I don't go too far by their dates!

Please create an account

To comment on this thread you need to create a Mumsnet account.