Difference in LMP and scan measurement

[e306]

Hi all. I had an early private scan done yesterday - not for any particular reason other than wanting to make sure all was okay. I thought I was 7 weeks and 5 days according to LMP. At the scan, they said I’m 6 weeks. They tried an internal scan but I couldn’t tolerate it at all so they had to do an abdominal scan in the end. They were able to see the gestational sac, yolk sac, fetal pole and detected cardiac activity. However, I’m still feeling very anxious. I don’t know how the dating can be so different or why. I very carefully tracked my ovulation, when we had sex etc so I know I conceived on 30 September/1 October and there is literally no way it could have been after that! The 12 day difference between LMP which suggests I should be 7 weeks and 5 days, and the scan saying I’m 6 weeks is making me worry something’s not right. Has anyone else been in a similar boat? Any advice, tips? I know I can have another private scan in a few weeks, but it feels ages away… Also - not really experiencing many symptoms which also makes me worry!

6 weeks is too early to be reliable for dating and would not be used to Chang an EDD (7 to 13+6 is used in preference to LMP)

Thanks, that’s reassuring!

A few millimeters in measurement can be the difference between 6 and 7 weeks gestation so if the scan was done abdominally, it may not have been very accurate. Transvaginal scans are the gold standard at this gestation to get accurate measurements.

The fact that cardiac activity was detected is a good sign- try not worry (although I know that's easier said than done). Also it is still quite early for symptoms too, I didn't get any until 8 weeks with my second and some women get none!

Thank you - really hoping it is the case of just error in measurement! Trying to hold on to the fact they saw cardiac activity 🥹 really wish I could’ve had a transvaginal scan but I have vaginismus which makes me struggle especially in an unfamiliar environment.

Thanks also for letting me know re symptoms - that’s reassuring!

I agree that because it was abdominal you can’t really rely on it that much. If it was internal I’d be a bit more concerned about the dates being out. But with it being abdominal there’s much more room for error.

Thank you!

What date did you ovulate?

30 September - I used clear blue advanced digital ovulation tests.

Your ovulation date indicates that you probably are around 7w 5d, although implantation usually takes place between 6DPO and 10DPO so that can cause some discrepancy. When did you get your first positive test?

I’ve not used the clear blue tests, do they show when you are actually ovulating or when your LH spikes? Ovulation usually takes place 24-36 hours after the spike. I’m only asking because if the clear blue tests show the spike in LH, and you had that on the 30th, you might not have actually ovulated until the 1st/2nd Oct, and then possibly not implanted until 12th Oct which would put you at 7 weeks.

All of that is basically to say that even when we think we are absolutely sure on dates, there are various factors that mean we can be off by 4/5 days.

As others have said, it’s really tricky to get accurate measurements at this stage (especially with an external scan!) and if the measurements are off at all it makes a big difference to the estimated dating of the pregnancy. I had an early scan at 6 weeks (internal) and they told me I was 5w 3 days. My 12 week scan dated me over a week later.

It’s really reassuring that they were able to see cardiac activity and the fetal pole with an external scan so I’d try not to worry about the dating at this point! Good luck xxx

I would put this down to the abdominal scan being much much less accurate than an internal one. Abdominal scans at this early stage aren’t recommended. Hearing a heartbeat is the strongest indicator of outcome - was the heartbeat strong and how many BPM was it?

I've had 4 losses and 1 baby. My living baby was measuring 9 days behind at my first early scan. But a heartbeat was detected. A few weeks later he was measuring 8 days behind. At the 13 week scan he was measuring 6 days late. He eventually caught up and ironically was born 50th percentile bang on his due date.

Also an abdominal scan this early isn't a reliable measuring tool as it can't quite accurately see!

Thank you so much for such a thorough response 🥹 my first positive pregnancy test was 11 October using the clear blue early detection pregnancy test. Have also done 3 tests since which show week progression and all have progressed well from 1-2, then 2-3, and then 3+ weeks!

The tests show the LH spike 😊

Honestly really appreciate your response - it’s put my mind at ease a bit more! xxx

It's tricky at this early stage as the actual fetal pole is so tiny even with an internal scan they can be off. That fact you've seen a heartbeat is great and don't forget that sperm can actually live up to 5 days so conceiving a couple days later than you ovulated is not within the realms of impossible. My first baby I had an early scan and my dates weren't spot on but by the time I had my 12 week scan my due date was only adjusted by 2 days. My second my due date was pushed back a week! That baby came a few weeks early but at a good weight and the midwives agreed my original dates were probably more accurate but they can only go off the scans.
It's annoying because it is unfortunately just a waiting game for you OP but don't be disheartened by measurements it's really common!
Congratulations too!

Hey op, just wanted to say I’m currently in the same boat. I had early nhs scans due to previous losses including ectopic. IVF pregnancy so my dates can only be a couple of days out due to implantation date being the only variable. My first at 5.5 was internal, fetal pole at 1.2mm, possibly the start of cardiac activity but got called back at 6+5 to confirm. They always start with abdominal, and she immediately saw a rhythmic hb so didn’t do tv scan. But she said the crl was 2.6, (which from everything I’ve read is too small). I’ve asked around the internet, and read a few papers, and the general consensus is that at that stage abdominal scans are not reliable measurements. The sonographer (who at my last appointment was very reserved about being positive) this time was cheery and said congratulations, completely unconcerned about the size and said I should not worry about it, that the two scan types aren’t comparable.

another point is that most papers agree that the hb isn’t generally visualised on an abdominal scan until the embryo is 7-10mm. So I’m taking that as hopeful that my embryo was actually sizeably bigger than the measurement as it was so immediately clear. I’ve another scan two weeks from the last for reassurance. Here’s hoping for both of us that pretty much everyone is right about abdominal scans, including the trained sonographers!

Thanks everyone for your messages - so reassuring! 😌

@Nosejug - really appreciate your comment, it’s put my mind at ease a lot! I think some of this for me is also about knowing I’m not the only one going through it, which has felt like it over the last couple of days. Keeping fingers and everything crossed for both of us - I’ll also be having another scan in a couple of weeks, and hope that things will be so much clearer for both of us! Best of luck ❤️

It ca be different because the embryo doesn’t often grow in a linear fashion. My current dates put me at an impossible time for conception, but it’s just how the growth is going.

And to you too @e306 ! ♥️

Also in a very similar situation OP... I went for a scan last Friday at 7w4d based on LMP. My (transvaginal) scan put me at 6w with a weak heartbeat. Prior to this, I went for a scan at 6w2d and was measuring 5w5d, so I sadly went in expecting the worst.

My prognosis on Friday sadly was not good and I have been referred to EPU on Monday. I think this was as it was transvaginal, it is a lot more accurate than abdominal. I'd be a lot more optimistic that things will work out for you. Keeping everything crossed for you and hope that you will have a positive outcome 💜When is your next scan?

I remember my consultant saying that if he saw a strong heartbeat during a viability/dating scan then he wasn’t worried - hope that reassures you slightly!

@e306 have you gone for another scan yet? I just wanted to give you some reassurance, as I was measuring a week being on abdominal scan (I told you above), but just had a scan two weeks later and am measuring a day ahead, so its absolutely true that abdominal scans aren't accurate very early on. Additionally, her first measurement put me 3 days behind, and she told me to empty my bladder a lot as it was squashing the sac, and when I came back it was now a day ahead. Thats 4 days added growth just by removing distortion from the bladder.

I really hope you get reassuring news soon 💚

How kind of you to check in ❤️ yes had another scan last night, and it didn’t go great. Again an abdominal scan of course - she couldn’t really see a fetal pole clearly this time though, or cardiac activity, and she was sure that at this point things should be clearer. I’ve not had any symptoms of miscarriage - yet anyway... Gestational sac was measuring as it should, though. Also found a little bleed (hematoma I think?)

As it was a private scan though, she couldn’t really be conclusive either way, so
they have referred me to early pregnancy assessment unit - but they won’t scan me for 2 weeks… which is just shocking, I feel anxious like never before. Can’t believe I have to walk around for 2 weeks not knowing if I’m growing a life or not, and potentially expecting the worst, it’s horrible feeling I wouldn’t wish on anyone 😞

So pleased for you and that you’re measuring ahead - that’s wonderful news! ❤️

@e306 oh pal I'm sorry you've not had a reassuring scan. I definitely would trust NHS over private scans in this instance, but thats an awful long wait. I also have a hematoma (I bled all last week it was horrible). I wish I could give you reassurance but I guess its just a wait and see. Im so sorry its such a wait though. Did they give a reason for it being so long?

Thank you 😔 yeah not too worried about the hematoma as I know it can be quite common - especially with my retroverted uterus which seems to be the cause of a few issues!

They just said it’s their policy, upon reviewing the private scan report, that if I was okay with an internal scan they would’ve done it in a week, but as I’m not, it has to be two weeks for an abdominal scan - it’s their policy apparently ☹️ as I’m not bleeding no pain etc, there’s no rush to get me in. I kind of get it but also feels mega unfair - it’s really taking a toll on me mentally 😢

ah yes I forgot you were unable to have an internal scan, though for a medical condition, I do wish they'd make an exception - it's not like it's simply preference. I actually have a retroverted uterus too.

At least the answer will be real by then, not that that's reassuring in itself, but if you were to get a scan in a week and it was inconclusive, you'd be waiting a further two weeks. By waiting two weeks first you'll know either way. I'll be thinking of you.

Yeah quite, I was hoping they’d be able to offer sedation or something along those lines - I’m not the only person in the world with that condition, so it’s not very thoughtful.

Thank you - I hope everything continues well for you with your pregnancy x