How accurate are 12 week scans?

[artysmurf]

So when I went for my 12 week scan I was 100% certain I knew my baby was under 12 weeks old I was certain I was like 10 weeks or something as I knew when my husband and I were intimate.
When they told me I was 12 weeks and 2 days I went home and counted back - I supposedly conceived 3 days before my PERIOD. I had a full heavy 1 week period- how could I have been pregnant whilst on my period?? It couldn't of been implantation bleeding surely as it was far to heavy it wasn't spotting at all.

Pregnancy is dated from the first day of your last period, which typically adds on 2 weeks before the baby was conceived, so that’s sounds correct.

I know about when pregnancy is dated from the first day of your last period but I thought they were supposed to give you an accurate age by the 1st scan

I'm not a doctor but thought My experiences might be of interest. My DD1 was born at 36 weeks according to her dating scan but a perfectly healthy weight (same weight I was at 41 weeks. I mentioned to a midwife that I dreaded to think how big she would have been if she's gone to full term and she said 'Oh, it's not an exact science - the dating scan might have been a cycle out'. Only one persons opinion and she could have been misinformed, but it did make me wonder. Dating scans are done to give a rough estimate of birth date, not to back calculate when conception happened. Anecdotally, I've seen quite a few threads on here saying conception couldn't have happened when dates imply because they hadn't had sex, too, so it strengthens the idea it might not be an exact science.

Conversely with DD2, I had period-like bleeding until the start of the second trimester so didn't know I was pregnant until then. That was down to a pouch on the c-section scar from the first but if I'd gone by bleeds, I'd have been well off.

If the dating scan was a few days out, it shouldn't make much difference to when your baby is born. Very few are born at exactly 40 weeks.

Hi OP I’m not a sonographer or medically trained but I think if I remember correctly that the dating scan is about the measurements they can achieve which they then use to date a pregnancy. In both of mine they put the baby “ahead” of date of conception- I knew the date I’d conceived. As do you most likely.
But it’s not about that for pregnancy purposes Perhaps as humans are getting significantly taller the standardised measurements need to change.
But it really doesn’t matter (there’s no true perfect accuracy in ultrasounds despite what we wish for - babies move and it is just bouncing sound waves etc) - I guess the real purpose for them is not precision conception dates but gestation expectations for a standard foetus.
They try to get the measurements of femurs, check spine, get predictors for trisomies etc.
It’s more like identifying an “11 week embryo” when it’s expected to be around 13 weeks (potentially growing slower - cause for concern?) or discovering an already 3 month foetus etc.
Originally that so-called dating scan was to screen for trisomy 21. If you bear that in mind it’s not about getting an exact date for conception and then counting forwards to due date but rather - if we expect a 12 week foetus and compare to a standard should we have concerns.

I think all the above is right. Hope it helps a bit. Congrats on your pregnancy!!

Scan dates are always 2 weeks more than date of conception. So scan dates = 12+3 means you conceived 10 weeks and 3 days ago. Measures will be give or take a couple of days.
all sounds like it lines up unless you mean you’ve counted back using the way I’ve outlined - in which case yes that’s a bit unusual but would chalk up to one of those things, and measuring bigger than expected is not concerning in the same way measuring smaller than expected might be.
congratulations

I don't know what this means. The "accurate age" is still counted from the first day of your period. That doesn't change.

If the scan was today and they said 12+2 that would mean the first day of your last period was about 23 June, with conception around 7 July with a couple of days adjusted being normal and expected. Does that fit?

My 12 week scan aligned completely with my last period even though I know for sure I ovulated late! Usually when it measures up to last period you will have ovulated around day 14. Maybe you ovulated early?

EDD date is more accurate from scan but the 1st day of pregnancy is counted from 1st day of last period throughout pregnancy. It’s only an estimate as in no one knows how long it took spermy to find the egg and then how long it took the egg to travel down fallopian tube etc. could be a few days either way

They do, but it still is dated the same way, so it’s correct.

Well good news I spoke to the midwife today for a routine check and she said they still add on the 2 weeks so if the scan says your 12 weeks your baby is actually really 10 weeks old but like you all said they count it from your last period. My brain can now relax from the confusion 😂