How accurate are early dating scans?

[HK1234]

Hi there,

I went for a private scan yesterday at what I thought was 7+4. It's my first pg and I've been a bag of nerves since I found out as I haven't had many symptoms so wanted to make sure, plus I have a large fibroid and wanted to check that it wasn't likely to cause any problems- thankfully it's not in a dangerous position at the moment.

Anyway, I had some good news in that a heartbeat was found, which was such a relief! We also found that I have a tilted uterus so the scan had to be done internally. However, I was dated at 6+2 instead of 7+4, but I'd thought my own dates were pretty accurate given when I found out I was pg etc. I may be wrong as I have a slightly irregular cycle but have a few concerns about whether it's that the baby isn't growing fast enough. I had accidentally only been taking 1 prenatal vitamin a day when the dosage is 2, which caused me to have a total meltdown when I realised on Friday so hoping that hasn't set things back.

I just wondered what others' experience of early scans were in terms of dating? I have read lots of different views so am confused. Sonographer said at this stage it"s give or take a week so hoping that's the case. Counting down the time til my 12 week nhs scan (when the appt comes through) and hoping for the best!

No dont worry. I had an early scan at GA 8+6 that dated me at 8+0. My dating scan gave me back those days and I measured 11+6 at 11+6.

Your own dates are never exact anyway so dont worry. I'm sure your little bean is doing just fine in there.

I don't think a scan is called a dating scan at 6 weeks is it? they can't accurately date a pregnancy that early. the most accurate time to date a pregnancy is between 11-14 weeks when the dating scan is carried out by nhs.

Thank you for the reassurances. I had a gut feeling that 6+2 wasn't right. You're right I don't think it's called a dating scan, still getting used to all the new lingo!

First scan at 6+5 measured closer to 6 weeks. Second scan at 10+1 and moved forward to 10+6.

So the baby more than caught up!

My NHS dating scan for dd2 put me a week further forward than the private scan I had at 9 weeks. Don't worry about the prenatal vitamins they certainly won't impact on baby's growth. If you can take them do obviously but baby will get what it needs from you,

Early scans are not correct for dating, especially with a tilted uterus, so early the tiniest miss-measurment can make a huge difference. I have a tilted uterus too and my early scans have never been correct, always around 5-7 days behind until the dating scan where it catches up.

I'm having an ivf baby- so I know my dates exactly. My early scans were out by 5 days (measured smaller), by my 12 week dating scan baby was measuring 2 days ahead- and still is at 31 weeks.
Early scans make measuring very difficult as baby is so tiny.

This is really good to know, thank you for sharing your experiences. It sounds very common for them to be out by quite a margin. I'm going to stick to my original dates for now I think (feels better to think I'm further along!) and all being well see what the 12 week scan brings. I'm in the dark about alot of this stuff as none of my close friends have had babies yet and we're waiting to tell our wider circle of friends til later. Wish you all very H&H pregnancies!

When did you get your BFP? Was it 3.5 - 4 weeks ago (consistent with your dates) or more recently than that? From my experience the early scans are pretty accurate (and also used to have a tipped uterus, which sorted itself out in first pregnancy!). A 7-week embryo is still absolutely tiny but it does look quite different from a 6-week one, and more than twice as long, so unless the sonographer wasn't very good I would take their dates as likely to be more accurate than yours. Nothing to worry about, provided you saw a heartbeat: it's just, in my experience, really hard to know when you ovulated. In last pregnancy the due date at my 6 week scan turned out to be more accurate than the 12-week date, though neither was the same as my due date calculated from LMP...

If you are thinking of having the NIPT test I would definitely urge on the side of caution and go with their dates, because if you go too early (e.g. at 9 1/2 weeks) they won't be able to do it and you'll have the expense of another appointment a week later. For the NHS, though, you might as well just carry on with the original date and see what happens at 12 week scan, where they can always put you 'back' if they need to.

I had a scan at around 7 weeks with DD1 that didn't agree with my dates, then had another two scans at 10 & 12 weeks that pushed my dates back by around 11 days in total. DD1 arrived 22 days 'early' according to these scans but looking at her, she certainly didn't look that early and the midwives agreed. Like everyone else has said, it's pretty common to have your dates changed by scans but it's normally nothing to worry about

Hopsalong my BFP was 13th Jan, so just over 3 weeks ago. I guess it's possible I may have been only 4.5 weeks by then, but the line was pretty dark and showed up quickly. Sonographer seemed very good and was from a reputable clinic so I think she knew her stuff but said it can be give or take 7 days. Was just concerned that things may be behind in development and a worrying sign, but we did see a heartbeat so will hold onto that fact and cross fingers. Yeah I'll definitely carry on with the LMP calculated date for the NHS as I may have to wait longer than the original 12 weeks if there is a backlog with appts anyway.

Also what is the NIPT test?

Thanks shippersss. Sounds like it's not an exact science and imagine babies must grow at different rates. It's a very confusing time these early weeks and time starts to go so slowly! Even potentially being put back my one week feels a bit torturous as you just want to get through the worrying early phase! I guess I will need to get used to the unpredictable nature of it all.

Did you get any negative tests before the positive one? If so, I reckon you ovulated a bit on the late side -- new date might not be right, but you probably aren't as far ahead as you thought (and so shouldn't worry at all about growth etc). I have with three pregnancies been able to get a BFP 9-10 days after ovulation, 2X 31/2 weeks into a 28 day cycle, and once 5 1/2 weeks into a very long one! This seems to be what the manufacturer's instructions assume. i.e. If the test is a fairly sensitive one, you are going to be at most 4 week pregnant when you get your BFP and probably less than that...

I had an early scan at 6+4 due to repeat miscarriages where they told me to expect the worst as I barely measured 5 weeks. My dates and my +HPT said 5 weeks wasn't possible and I cried for hours. HCG rose acceptably though and I returned when I thought I was 8+2. Again, I measured 2 weeks behind at 6+5 but there was a lovely heartbeat. I was still so upset though as I couldn't possibly be 2 weeks behind and thought something must be wrong. I'd only dtd a couple of times around ovulation!
Another two weeks later revealed I was now measuring 9+2 weeks and a baby could be seen waving its arm buds! Incredible! Felt better because at least I was pretty much consistently behind.
A reassurance scan at a private clinic at 10+4 (the behind dates) suddenly put me at exactly when I thought I'd ovulated- 12+2! Dating scan confirmed it at 13+6 and our sexing scan and the 20 week scan are all consistent for my early dates. I'm now nearly 22 weeks and everything is going textbook well.

I don't know why I kept measuring behind so early on. And so consistently behind. Maybe because it's too early to be accurate, maybe because I'm overweight, maybe because I have a slow starter (makes the most sense given her father and sister in the mornings ) but a little research seems to be that this can be the case for many, many women.

Glad to hear that everything turned out well in the end for you StrawBrhi. That must have been a really scary and confusing time waiting for your next scans.

Hopsalong no I hadn't done another test before as I'm often late and assumed what turned out to be my early pg symptoms were PMS. Clueless! I imagine the dates must be somewhere between the LMP calculated date and the scan date.

I don't think they can get it spot on (even if they claim that environmental factors don't apply before 14 weeks, and every baby is exactly the same length on a specific day of pregnancy).

Going by my LMP I would have been 12+3 on the day of my dating scan. I thought I was no more than 11+6 (if I conceived the same day as I got my first positive OPK test). The sonographer put me at 12+1. Meaning I apparently conceived the day before we had sex for the first time that cycle.

I still think that I'm a few days behind my official dates but I'm having to go with the what my notes say. I just hope it doesn't mean getting induced earlier than absolutely necessary should I go overdue.

Saying that I've had an extra scan (went for a CVS last week) and strangely I measure exactly on-track from the dates in my first scan. Maybe it was an immaculate conception after all!