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can early pregnancy scans get dates wrong?

(15 Posts)
pep124 Thu 15-Sep-11 20:39:42

Hi everyone

I went for a private early pregnancy scan for a number of reasons today (very complicated personal circumstances, my age, lack of any real pregnancy symptoms - and I'm just a worryer!). I was very certain about my dates, as my boyfriend lives abroad it narrows down the possible conception date!
* 18th july was first day of last period
* since then have only seen bf twice - 30th july and 6th aug
* got BFP 17th aug
I therefore was certain I was 8 and a half weeks pregnant.
But when I had the scan she was adamant I was 6-7 weeks. She saw a heartbeat and said all looked healthy and normal but she was certain about the dates.
So - from the above do you think it's at all possible her dates are right? And if not, could it mean something's wrong and the baby isn't growing properly? Or are the dates at this point inaccurate?
For what it's worth, not sure if this makes a difference, but my cycle was always between 22-26 days each month.
Thanks for any help from anyone better at this whole pregnancy thing than me - or better mathematicians!
Kate x

feekerry Thu 15-Sep-11 21:03:03

Congratulations. Well when I had an early scan I would have said I was about 8 weeks but they said just 7 after early scan. When I went for my 12 week scan they said then that I was more like 13 weeks and moved my date forward again in line with my original thoughts. They said at my 12 week scan its a much more accurate date wise than an early scan.

banana87 Thu 15-Sep-11 21:23:26

Congratulations! When I went for what I thought was a 6 week scan in this pregnancy, they told me I was more 5+4 and could only see a fetal pole (no hb). I went back a week later and they saw a hb. 2 weeks later they moved my date forward to what I originally thought! So yes, it can happen. Early scans are highly inaccurate as they are measuring something so small and just a little twitch can put things out.

notlettingthefearshow Thu 15-Sep-11 22:28:42

You have to wait for the 12 week dating scan - this will be more accurate.

TheDetective Thu 15-Sep-11 22:32:06

Agree with the above poster - wait for your dating scan - that will be the date to go from, and you will probably find it correlates with your own dates more then.

BikeRunSki Thu 15-Sep-11 22:34:44

Congratulations. I had a scan at what I believed to be 9 weeks to the day. Scan made me 7+5 or something. 12 week dating scan agreed with my dates perfectly.

pep124 Thu 15-Sep-11 22:52:10

Oh thanks everyone. She was totally adamant and didn't say they was any room for error - even when I started giving TMI about only having had sex on two nights since july 18th! Am just going o be reassured about heartbeat and be patient until oct 7th (nhs scan).

BonzaBlue Fri 16-Sep-11 02:57:13

But the sperm can hang about for a few days and you can ovulate a little later than when you had sex ... DH and I dtd on day 19 /20 of my cycle - and according to my LMP i should be due on the 4th of Jan but my first scan put me a week later ! So now due on the 10th and all my following scans have said the 10th so we will see !

kat2504 Fri 16-Sep-11 06:11:26

I think you probably ovulated a few days later this month. When you got the BFP was it a faint one or a dark line? Were you testing because you thought you were already late?
Early scans at that stage can be up to 5 days out. I had one at 6 weeks that was 3 days out, but by 9 weeks I had got those three days back again.

So if the scan is 5 days out, and you ovulated a few days late, and those sperm were super long life swimmers that waited around, it is perfectly possible for your date to be about 10 days out.

As to whether or not something is wrong, only time will tell that. For the moment the sonographer has given you no cause for concern and said that everything looks good on the scan. Seeing a good heartbeat is always reassuring. Have you got a re-scan booked?

spannermary Fri 16-Sep-11 08:06:28

This is a very interesting thread! I thought I was 5/6 weeks but my early scan dated me at 8. Everyone else seems to have had the opposite experience. I thought I'd had a period towards the end of July but they reckoned that must have been implantation. It was quite heavy. '12 weeks scan' in 2 weeks when according to their dates I'll be 12 1/2. I'm wondering if it's too early. Could they think a 5/6 week peanut was 8 weeks? That seems like a big difference! (would explain my high HCG at that time though).

kat2504 Fri 16-Sep-11 08:30:09

No no a 6 week peanut is barely visible and looks like a squiggly seahorse. An 8 week peanut is 3 times the size of a 6 weeker and looks like a little jelly baby with tiny little arms and a big head.

pep124 Fri 16-Sep-11 10:02:10

Right, called the clinic as was getting in a right old pickle - having a bf who lives abroad makes you fairly certain on dates! They've just explained - which wasn't explained last night, even when I questioned it - that they give a date as weeks since conception. Therefore measuring a peanut gestation of 6-7 weeks (ie since conception) also equates to me being 8-9 weeks pregnant - all ties up perfectly. Phew.


spannermary Fri 16-Sep-11 16:47:04

Yay - glad you got it sorted! As for my peanut...looks just like a jelly baby! Blimey, MNers sure know their stuff. smile

Jezzabell Fri 16-Sep-11 18:24:03

spannermary I had a similar experience at my 12 week scan. I went in thinking I was 12+5 and was told I was 13+4. Going by the scan dates I was pregnant 5 days before the first day of my last period (4th June). I know that my last period was definately not implantation bleeding as we didn't dtd until 15th June and it had been a good few months? weeks before that time and we would have used a condom. I've worked out my own due date by counting 266 days (length of gestation from conception) from the earliest possible conception date and it comes out as 3 days after the scan date and 5 days before the first day of last period date so I reckon I'm going with that!

I've got my 16 week MW app on Tuesday so I'll ask her about it then as it's a bit confusing - I couldn't have been pregant before my period!

themightyskim Sat 17-Sep-11 10:43:48

well this has confused me lol, I had an early scan at what I thought was 6 weeks and they dated me as 4+6 so I worked from that date then, but if thats the date from conception then I would have been 6 weeks but there was no fetal pole or heart beat etc and she said that was perfectly normal and in line with my dates, so perhaps not all go from conception date?

Im going for another scan on Tuesday as Im nine weeks with still no symptoms and the few Ive had have gone, I suppose if alls well then Ill have to be asking how they get their dates lol

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