Dating confusion after scan!

[Ekphrasis]

I had a private scan last week linked to harmony - I’m guessing the equipment isn’t as accurate maybe? I was pushed forward 8 days from what the nhs lmp is and 4 from what I’d guessed as I tend to have 25 day cycles. But I can’t believe that the new date is right as it would be the immaculate conception!

I was moved forward a bit with my son but not by as much. Can they have growth spurts in the womb? I have my nhs scan this Thursday so I guess it will be more accurate- I hope!

They don't date it from conception its meant to be dated from the first day of your last period which is why i dont get why they move it? At my NHS scan they moved my date back 4 days which means i would have conceived in the middle of my period if they try and predict conception date. It's all very confusing as they date it so that the first two weeks you're not even technically pregnant

I was given a date 6 days behind what I’d worked out from ovulation at 8 weeks and then my official due date from 12 week scam was 4 days ahead (so 10 days ahead of first scan!). There is no way she could have been conceived by those dates but I just went with it!

It is based on your first day of your last lmp (last monthly period). Pregnancy /conception calculator are usually based on having a 28 day cycle. Since you have a 25 day cycle it will give your due date as slightly earlier. The scans use the measurements of the baby to decipher the due date but the baby could have been in a slightly different position for each scan.

Babydreams, I understand that, I should have explained better.

If the sonographer put me at 11+ 5 based on cr length, according to uk measurements conception was 75 days ago (iirc) which is impossible. The lmp (28 days) was putting me at 10+4.

Different position seems likely. I’ll be trusting the nhs one better, she just seems pretty convinced and is an nhs one during the day.

It’s making my head hurt so I’ll just go with it! Sound like my dates based on 25 day cycle are more accurate.

I was moved forward 4 days which wouldn't have coincided with when we dtd (one try, one pregnancy) but babies grow at different rates. At a growth scan i had they measured me again and I was showing another 4 or 5 days ahead of what I was despite them being worried that he was on the small side.

The lmp dating is based on cycles when you ovulate on day 14. So if your cycle is different, that’s some discrepancy. In addition, a released egg doesn’t necessarily get fertilised right that second son there could be another day of difference. And growth rates vary - if you look at IVF, some embryos are 1 day ahead of others by 5/6 days after fertilisation! So by a few weeks of growth there well may be a much bigger difference.

I once read about this lady that went through IVF and had twins. Obviously both fertilised at exactly the same time but in the third trimester one measured 3w(!) ahead of the other. Both were ok in the end.

I quizzed the (NHS) sonographer after my last dating scan as the due date given didn't match my dates and as I wanted to try for a vbac it wouldn't have given me much leeway as my first pregnancy went post-dated and was told that they were accurate +or- 7 days.

Ok thanks, so it’s a pretty inaccurate (by around a week) science!

I’m pleased it’s not the other way as I was worried about growth following a mc and also a low birth weight son. He didn’t start to loose weight / size till later though.

Ekphrasis, sounds like this baby is doing very well indeed! I hope you don't stress too much over it and best of luck!

At the scan with my harmony test I was put forward 8 days! I knew my real dates as we were ttc with OPK’s and temping etc. By 12 weeks at the NHS scan we were moved forward another day, so 9 days in total, it’s all done on length of baby and I have tall kids! My 4yo is the height of a 7-8 year old!

Dating is most accurate around 12 weeks. So if you are in fact 10+4 by lmp, it could be just an inaccurate measurement because baby is still so small. With my first, my dates were changed by 4 days at dating scan. But it doesn't mean you necessarily conceived that early. It means that's how baby is developing and things like early or late implantation can make a difference. My first implanted early (I had very early symptoms and cramping) and got early positives, so makes sense given my dates were moved 4 days earlier. My 2nd implantation was later, I think, around 10 dpo, and I've only moved 2 days earlier this time. Also, it's totally down to the skill and care of the sonographer. Some are more accurate than others or take more time with it. Being off by a fraction of a mm can make a huge difference. The measurements are much easier to get the bigger they get, so you may see that by 12 weeks you get a measurement that is more in line with your lmp.

My first DC dates were moved by 27 days from LMP as I don't have regular periods and nobody knows where I ovulate in my non-cycle. I wouldn't be too concerned about being moved a few days - sperm can take a while to reach the egg anyway so even with an exact LMP and ovulation you could still be a few days later than you might think.

Scans aren't to figure out when your baby was conceived - for their purposes that's completely irrelevant. They're to figure out when your baby is sufficiently developed to be considered "full term" ie healthy enough to survive independently, and/or when they are most likely to be born. They look forward, not back.

Your definitive due date and gestational age come from your 12 week scan because this is the point in the pregnancy when it can be most accurately established. Before that baby is too small for high accuracy, after that growth rates vary and so size doesn't accurately reflect gestational age. Until your 12 week scan your dates are officially based on LMP as the best available rough estimate. And yes, they are accurate +/- 7 days as there is natural variation even when date of "conception" (ie fusion of egg and sperm) is precisely known. Bottom line, the private scan may or may not turn out to say the same as your 12 week scan, it's all really just for interest until then.