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When dates don't match up

(16 Posts)
VaselineOnToast Thu 10-Apr-14 10:37:26

I had my dating scan right on the day my 12th week started. I was completely certain about the dates since my periods are very regular and I always keep a note of the start date. However, at the scan they told me that baby was measuring for exactly 13 weeks! Counting back, that takes me to the week before my last period started. Makes no sense!

How does this work? Does it just mean that my little foetus is growing particularly quickly or what? What if it measures less than it should at the 20wk scan since I'm technically a week behind what the sonographer thinks?? I'm very confused!

VaselineOnToast Thu 10-Apr-14 10:45:02

Thanks in advance for your comments! smile

slightlyinsane Thu 10-Apr-14 11:55:50

Scans aren't precise and usually come within a5ish day inaccuracy. My 12 wk put me at a wk ahead which I know can't of happened. I have fortnightly scans and they have always measured ahead of the adjusted date aswel. It's just one of those things that happen. My other 3 all measured ahead aswel.

Vatta Thu 10-Apr-14 12:06:35

Don't worry, scans aren't that precise. A friend had ivf so knew EXACTLY when the baby was conceived, but her scans always came back a week behind. They're only accurate to within a week or so.

CaseyRossi Thu 10-Apr-14 12:09:32

i was just like you, 100% 11wks 5 days at scan and they dated me 12+5 which i know isn't possible, i said could it be down to my short cycle? i was having 23-25 day cycles for the 3 months after coming off pill and i also think i may have ovulated around day 8, they said it could be.

I hated it as i lost that "yay 12weeks" moment, but it's okay now, means i get my 20wk scan a little sooner, will find out on the 25th if dates are still odd, when people ask i started off saying 13 weeks but actually 12 weeks but now i've got used to having to use the nhs dates as those are the ones they base appointments on.

I wouldn't worry too much if its up to about 5 days out - I had IVF so could not have been any more certain about my dates….I knew almost to the minute when that embryo was created and implanted!
However at my first scan (7 weeks) it measured one day behind where I was, second scan (11 weeks) measured 5 days ahead, 3rd scan (12 weeks) it was still 5 days ahead and they tried to adjust my due date (which I was having none of!) then the 20 week one (which is less accurate for dates) each body part (legs, head and abdomen measurements) were all bigger or smaller than the actual dates.
I'm fortunate that I know my dates exactly and there is no possibility of short cycles, me getting dates wrong etc for me - I knew what my due date would be before I was even pregnant but in your situation I wouldn't worry too much.
Congratulations btw!

hubbahubster Thu 10-Apr-14 12:41:14

It tickles me that hospitals seem to expect all babies to grow at a set rate in utero – no one expects that their kids will all grow at distinct times once they're born, do they?

DC1 was measured a week ahead at the 12 week scan, DC2 five days (even though my cycles are like clockwork so I know exactly when each of them was conceived). Don't worry too much, it's a bit arbitrary and I'm mentally dating DC2 by my own dates anyway…

VaselineOnToast Thu 10-Apr-14 16:21:36

Thanks everyone for your comments! I feel reassured now. There really is no way that all babies are going to develop at exactly the same rate, you're right. I'm glad you've all experienced this strange phenomenon as well!

Mummyto3tobe Thu 10-Apr-14 16:42:48

if you were measured a week or so ahead of what you thought you were can i ask did your baby arrive early/on time or late?

whereisshe Thu 10-Apr-14 16:56:49

Actually at <12 weeks babies do develop at almost exactly the same rate. That's not the cause of the date difference.

What is more likely is :

- the error margin of a scan at that gestation (+/- 5 days due to the equipment not being perfectly precise and the baby moving about etc),

- or variation in when you ovulate (knowing your LMP and cycle length doesn't automatically mean you know your ovulation date),

- or slight variation in fertilisation / implantation timing after ovulation

Anyway, the most important thing is to remember that when they say "due date" it has this level of variation, in addition to the length-of-pregnancy variation. So any HCPs later on insisting on induction due to post dates are really talking out their arse <steps off soap box>...

mistlethrush Thu 10-Apr-14 16:56:50

I was certain of my dates - first scan (6 wks) was +1, next (8 wks) + 2, by 12 wks +3 etc etc. So my dates changed all the way through!

ithoughtofitfirst Thu 10-Apr-14 19:14:06

I was 13+1 in my first ever scan... you're measuring 13 weeks and one day they said. Bang on.

This time i was 5 days out.

I don't think it means anything. Unless you're like a month out. Then you definitely counted wrong.

Fwiw dc1 had a 'massive head' off the charts all the way along but was a dainty 6 lb 11.5oz born.... 2 weeks overdue. It's all a load of crap basically

Boogles91 Fri 11-Apr-14 13:28:33

I had what i thought was 8wk scan, and it put me at nearly 10wks, then 12wk one showed i was 14wks! So i dunno how that happened or wich is correct but i cant even remember exactly when my last period was...but i know i wouldnt have gone two mnths without one as we was trying for baby so i know i had one in october at some point and caught out sometime after. Babys fine at 20wks scan and thankfully didnt decide i was even further lol got another scan in two weeks. Wich is at 28wks cant wait to see lil man again :D x

mistlethrush Fri 11-Apr-14 13:32:01

Ithough - DS was also massive. And he was. 10lbs 5oz a week early! (and no rings of fat - just long, solid baby)

Misty9 Fri 11-Apr-14 13:33:39

With ds I was put forward by 5 days at my 12wk scan and he came....5 days late. With this one, I was put forward 3 days at the scan and currently 40+2 and counting grin

Let's just say we don't do the necessary that often therefore I'm pretty certain of conception date both times.

ithoughtofitfirst Sat 12-Apr-14 22:18:07

mistlethrush shock !! Your poor fanjo. Although i've heard their 'easier' to labour?! Big 'uns i mean. I think i'm having another dainty one this time.

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