How accurate are dating scans? Any charters out there?

[BabyValentine]

Just had my dating scan today and everything is fine [over-the-moon emoticon]!

I was charting for a few months while TTC as my cycles were a bit haywire. The month I conceived, my temperatures (and FF) say that I ovulated on CD15 (23rd May). That makes me 12+4 weeks.

But the midwife today has put me forward to 13+3 weeks. Meaning that I ovulated on CD9, right? That isn't particularly unusual as I had had previous cycles with ovulation on CD10, and CD20. So it is possible.

BUT, my other signs (EWCF and ovulation pains) suggest otherwise. My ClearBlue Fertility Monitor gave me a Low reading for CD9, and DH and I didn't even start SWIing until CD10.

Could the dating scan be wrong? I know it isn't particularly important, but I really found charting helpful and trusted my temperatures implicitly. Was I wrong to worship at the shrine of TCOYF?

For anyone similarly obsessed interested, my chart is here

Thanks ladies

Hey BV! I'm glad all is well with you and your little one. I've got my scan next Friday.

I'm also a bit puzzled on this dating malarky.

I thought you took your date of ovulation, counted back 2 weeks and considered that the "start date" to count from. However I suppose what none of us know is when fertilisation/implantation took place.

I told my MW my cycles are short, about 22 days on the one I got pregnant (ov'd on CD12, have 10LP), so she "adjusted" my LMP from 31.05.10 to 24.05.10 which fast forwarded me a bit. I don't really understand it but will be very interested to see what the scan says.

There are loads of folk on here who reckon their dating scan was wrong (i.e. those who have DHs who work away and dating scan gives a date of conception when he was abroad!). So it's definitely possible for it to be a bit out.

I guess it's only important at the end if they were pushing for an induction and you only thought you were 41 weeks.

Hi spilt! The induction thing does concern me a little TBH, but 'tis nothing in the grand scheme of things I guess.

But, 6 days is rather a lot I thought. We refused a NT and we must've only been in the room for about 4 minutes. The bedside manner of the midwife left a lot to be desired and the photographs are rubbish (face on as baby was lying vertical towards my bum, apparently). So I feel a bit cheated really.

I agree with your midwife's calculation of your EDD based on your LMP - your cycle that month would've been 22 days, but the LMP calculations are based on a cycle of 28 days, so you need to add 6 days, IYSWIM. Hurrah, lets hope the scan agrees.

From what I understand dating scans at 6-8 weeks are very accurate - with DD had one at 7 days and gave same EDD as my temp chart.

However by 12-14 weeks can be as much as a week out.

re: DH's abroad, sperm can live for up to 5 days waiting for ovulation so thats a factor if basing on "the act" rather than temp rise.

if you get to the point you are discussing induction you get your chart out and say you never agreed with your EDD....

Thanks Mum. I am leaning towards believing my own dates. There was no SWI'ing prior to CD10, so no super sperm possible...

Hmmmm i wounder this also, my 1dc was a slight accident so me and hubby didnt have a clue really as i was yound didnt keep track of periods etc.

Anywhoo my original EDD was 28th feb, scan said 7th feb.

When he was born 12 days over due, even the midwife said this baby was defenetly more over dude than 12 days she rekons another week or so.
I agree to, looking back for my first baby i felt him move at 14 weeks which i though was early for a 1st pregnancy.

i've just had my dating scan at 14 weeks and they put me a week ahead of where i've worked out i am, to 15 weeks. i don't chart and have no idea about lmp or cycle length but know my date of ovulation, and we did the deed on the day, definately not the week before. i also had a very early scan (not nhs) which backs up my dates.
my last 2 babies came pretty much on my due date (night before and morning after) which i find a bit wierd and those edd's were moved forward a few days each at the dating scan. when they were born they had a lot of vernix and the midwives commented they looked a bit earlier than my dates, reassuring me that my dates are right and i just have big babies!

Neither of my dating scans have agreed with my FF ovulation day. Only out by 1 day with this pregnancy, but 3 days last pregnancy - and baby arrived three days late - ie when I thought he was due!

Hi,
I had been charting for 10 months when I conceived last November. So although my periods were always irregular, I had a pretty good idea of when I ovulated, though not 100% sure as slightly ambiguous pattern. But what is clear is that my EDD should be either 17th, 18th or 19th August.
The 12w-scan put my EDD at 14th August.

I'm definitely holding on to "my" dates. My MWs won't hear anything about it and wrote 14th August into the space for "agreed EDD" although I don't agree. But when it comes to questions of induction and such I will simply insist on my own dates.

I have been supported in this by a MW friend. She explained to me that on average, boys grow quicker in the first trimester than girls, so they are on average a bit larger at the 12w-scan. But the dating statistics don't (can't) take account of that but use the overall averages instead. As I am expecting a boy, this would fully explain the dating disagreement. She told me not to worry if I went overdue and to insist on my own dates.

From some random research on the issue I gather that in case of disagreement, HCP are advised to go with the scan date rather than with the women's self-professed date. The reason for this is that over all, women are more likely to be off than the scan. Because women can be mistaken about their LMP, about their UCL, about their intercourse dates, about when in their cycle they ovulate, etc. But YOU are not the average woman. You know how sure you are of your dates. So the likelihood of YOU being mistaken is much much lower than the likelihood of the average woman being mistaken. When it comes to induction and such, you will just have to fight your corner.

Anyway, congratulations on your pregnancy, I hope it continues to go well!

Congrats OP. There has to be a margin of error for both your own observations and the scan.

I was told at all of my 12 week scans that the scan EDD can be out by up to 5 days each way.

I dont know how accurate temping is but IME ovulation pain and EWCM can occur over a period of up to 5 days around ovulation time.

Thanks everyone for all your replies. I am going to mention it to the midwife next time I see her and see what she thinks, but heyho, 6 days isn't massive in the grand scheme of things.

Still feel a bit cheated though with regard to the hurried scan, miserable midwife and rubbish photos. Anyone know how much a private dating scan costs?

I felt my scan was totally accurate as Midwife gave me an EDD of 07/08/10 (tomorrow!) based on LMP - even though I told her I did not have a 28 day cycle. Scan agreed that it looked like I had 33 day cycle that month and revised my due date to 12/08/10. Meant we had to go back for a second scan as I was only 11+1 when I first for first scan, baby was 1mm too small to qualify for NT test.