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how accurate are 12 week scans?

(36 Posts)
sarahjacksonxx Wed 16-Mar-16 18:46:44

Hi there!

This is my first pregnancy, i had my 12 week scan yesterday that showed i was 12 weeks 6 days making me 13 weeks today although that takes me back to the 16th dec and my normal period actually started on the 19th dec i have a normal cycle 26-28 days every month! i was just wondering how many days scans can be out by? Was told my due date is the 21st September with that due date i was also wondering if there was any way at all i conceived one or after the 9th jan making me ovulate late in my cycle or would an ultrasound pick up if i conceived later in my cycle? I'm so confused lol, i don't expect anyone to know the answer just a little bit of help would be great smile x

DontKillMyVibe Wed 16-Mar-16 18:52:07

They can be out by 1-2 weeks I think. It's the 20 week scan that they use as the dating scan and 'official' due date

mrschatty Wed 16-Mar-16 18:56:25

I thought I was nearly 14w when I went for my 12w scan. That scan dated me at 12+3 so 1+4 ish out from my period date. I gave birth at 38+3 so really it's all over the place and no correlation in my experience though I was a bit gutted that the first scan pushed my due date further away in the first instance...but obv didn't matter in the end as my dd didn't want to wait!

Broughtitatthe5anddime Wed 16-Mar-16 18:56:50

It's definitely the 12 scan week for dating. I was put back a week in each of my pregnancies but they will only use the dates from the scan anyway, the 20 week scan is for anomalies.

sarahjacksonxx Wed 16-Mar-16 19:07:23

I thought it was the 12 week scan also! thank you for your replies, does anyone think there could be a chance that i conceived on or after the 9th jan ? by the due date 21st september, my period started on the 19th dec my next period was due on the 14th-16th jan that didn't come ofcourse! i'm thinking maybe i ovulated later than usual but i do have a normal cycle, & would my baby be messuring smaller than it is if i got pregnant on the 9th or just after ? Sorry if i'm confusing people as i'm confusing myself lol.

SpeakNoWords Wed 16-Mar-16 19:10:13

Your scan date only makes 3 days difference from your estimate, I think? That seems normal to me - my dates were adjusted by about that for both my pregnancies. There's going to be some variation in when you might ovulate, and then when that egg might have been fertilised. Your cycle varies by 2 days as you've mentioned, which could account for the difference.

DontKillMyVibe Wed 16-Mar-16 19:47:05

Maybe it's different in NI. I know the 20 week scan is the anomaly scan but my midwife told me that's also the official dating scan whereas the 12 week scan is the rough draft dating scan grin I'll find out next week anyway as that's when I have my 20 week one smile

SpeakNoWords Wed 16-Mar-16 20:16:00

In England, certainly where I am, the 12 week scan is definitely the dating scan and that's the date that is used from that point on. Lots of things seem to be different in NI than in the rest of the UK.

Out2pasture Wed 16-Mar-16 20:20:49

congratulations op

the earlier the scan the more accurate the date. as time goes by the size and measurements are affected by the placenta, parents background, etc.

Skinny9741 Wed 16-Mar-16 20:56:30

I'm exactly the same but around 2 weeks behind you. I havent had 12 week scan yet but had private scans. I was told by posters on here and my sonographer that pregnancy is actually 38 weeks and they add 2 weeks onto whatever date you think you conceived. I still think this is a bit of bs but no other explanation

baby3forme Wed 16-Mar-16 20:59:29

Your exactly the same as me !!! Light period 16th Dec full period 19th Dec I conceived on 30/31st Dec in Cuba .... Bubs is due 21st September x

PurplePotatoes Wed 16-Mar-16 21:00:15

In my last pregnancy the 12 weeks scan pit me forward by 1.5 weeks from what I thought and I was pretty sure of ovulation date as I was obsessively charting temps etc. I did then go overdue by 10 days and have to be induced so I do.think my dates were the right ones.

Skinny9741 Wed 16-Mar-16 21:07:59

baby3 I conceived 29th Dec and not due till 5th Oct by scan, do you mean from your lp or am I totally confused AGAIN

KayTee87 Wed 16-Mar-16 21:10:07

It makes you 12+7 today, 13 tomorrow.

It's not likely that you conceived on or after 9th January and due 21st September as that would make gestation around 36 weeks and it's usually around 38 and it would mean you ovulated very very late in your cycle. It's more likely you ovulated around 30th December.
However it's all just an estimate and can be out. What makes you think you conceived on the 9th? Ovulating on the 9th would mean only a 5-7 day luteal phase and I'm not sure that's possible - maybe it is.

ShowOfHands Wed 16-Mar-16 21:10:23

You conceived around New Year's Eve. Is there a reason you're concerned? 3 days out isn't unexpected. 12 week scans are fairly accurate but with a margin of error of around a week each way.

Skinny what is bs? The way they date it? It's perfectly logical really. We didn't used to have scans or as much knowledge so pregnancy was dated from the date of your last period. So in an average woman, 2 weeks before conception. We've just stuck with what we're used to.

baby3forme Wed 16-Mar-16 21:15:08

By my scan x

Dixiechick17 Wed 16-Mar-16 21:26:38

skinny they count the two weels before you fall pregnant because they originally date it from lmp, and include the lead up to ovulation. Dating by the lmp gives them a rough idea of how far along you are prior to having the scan. So yes, actual length from conception is 38 weeks, but as far as a timeline goes and what tou read about at each stage, it includes those two weeks and takes the non pregnant weeks into account with measurments etc.

OP I was told 13 weeks at my dating scan which meant I would have conceived three days before our happy little accident. My reassurance scan at 8 weeks was actually more accurate and my DD arrived on the due date given at that first scan, which fell inline with when I would have ovulated smile

Skinny9741 Wed 16-Mar-16 21:37:26

Yes I know they calculate from lmp but thought that was only until first scan. I think it's bs as from my scan I conceived during my period when I didn't have sex that day or for about a week before hand. Thought I conceived on 16th Jan as that's the only time I had sex during ovulation and 27th and 28th Jan when I po lots of s some were still negative and some were very faint and pregnancy was only confirmed by blood test. I'm still non the wiser but happy I'm further on than originally thought smile

Skinny9741 Wed 16-Mar-16 21:39:10

Day I conceived by scan was actually 27th Dec.
Ps, sorry for hijacking your thread sarah!

KayTee87 Wed 16-Mar-16 22:18:15

Skinny if your scan said you were 12 weeks then you conceived 10 weeks ago approx. 13 weeks then you concieved 11 weeks ago and so on. Sorry if you already know that and I sound patronising in any way.

KayTee87 Wed 16-Mar-16 22:19:25

When they date you at 12 weeks theyre not saying you concieved 12 weeks ago.

cluelessfirsttimer1 Wed 16-Mar-16 22:49:09

Are you basically trying not to find out if you conceived in 16th December or 9th January? Just in trying to work it out but I'm getting confused over the dates. Did you have sex between those dates? I know I fell pregnant then had a 'period' which actually turned out to be light implantation bleeding as my scan put me a couple of weeks further than I thought. Sorry if this seems like a nosy question to ask. But are you trying to figure out who the father could be to the baby?

cluelessfirsttimer1 Wed 16-Mar-16 23:01:44

I'm not judging you by asking that. It's just that if those are the only 2 dates rhat u know u had sex, there is a hell of a lot of difference in a scan at this stage if the possibility conception dates are 4 weeks apart from each other

Sootica Wed 16-Mar-16 23:02:05

That sounds about right as the 13 weeks is counted back to the first day of your period, not when you thought you conceived. So scan is only 3 days out from period date.
The scans have a +\-7 day margin of error I was told by a sonographer.

MadameJosephine Thu 17-Mar-16 23:05:45

Dating scans at 12 weeks can be a few days out as they assume the fetus is on the 50th centile but not usually as much as a week.

Remember that the date of conception may not actually be the day you had sex, sperm can live for up to 7 days so fertilisation can happen later

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