Dating pregnancy and accuracy

[ApplePie86]

Wonder if anyone has any experience or knowledge of how accurate dating a pregnancy is?

Situation is that I am 100% confident of when I ovulated (based on OPK, temp and ovulation pain). There's no doubt there at all.

Because I have VERY irregular periods (I have PCOS) I can't use LMP for dating so worked out dates from ovulation and added 14 days (I actually ovulated on CD28 so would be way off!).

I had a scan when I would have been 8W2D and from measurements they said I was 8W3D. 1 day difference so no big deal especially when the actual difference is probably a millimetre.

I had another scan yesterday when I would be 11 weeks exactly. Sonographer took measurement and confirmed 11 weeks but baby wasn't in best position and wasn't moving.

A few minutes later it started jumping about and moving so she remeasured and said 11W3D. She was confident about that one and deleted other measurements.

Now, due to PCOS and therefore insulin resistance problems I assume I'm at an increased risk of gestational diabetes which would cause the baby to grow bigger than expected.

At this early stage could this already be the cause of why the baby is measuring bigger than it is?

These were private scans. When it comes to NHS will they simply ignore my 100% confirmed ovulation and dating and only work on dates with a measurement? Which could bring EDD forward and therefore not allow me to hang on as late as I may like to (if possible).

Any knowledge/experience with this much appreciated!

They’ll work it out based on your dating scan at 12 weeks because babies grow at different speeds before that.

Plus, while you know when you ovulated, you don’t know when it implanted - so you could be a few days out easily. They even use dating scans for people who’ve had IVF. By 12 weeks I was dating spot on for my tracked ovulation so my EDD hasn’t changed, but I was warned 4 or 5 days either way would be normal.

I was tested for GD at my 12 week scan, so you’ll know early if that’s had any effect on baby’s growth.

Remember only 4% of babies come on their EDD. Coming two weeks early or two weeks late is considered normal. If you go overdue; you’ll be able to talk about options and recommendations depending on how you both are etc

Thanks @TakeYourFinalPosition !

That helps loads.

Just paranoid that if baby keeps getting bigger and bigger, by the time I'm 38 weeks, they'll measure me at 40 weeks and want to induce when I'm actually only 40 - I would be delighted to hold off until 41-42 weeks although I'm sure I won't be saying that nearer the time.

They will work to the date from your 12 week scan but ask her to take a few measurements and use the average.

If you go ‘overdue’ past your due date then you will be offered additional monitoring (scans, check up with midwife) but if everything looks fine then you should be able to follow the course you want to take. They can’t force you to be induced (they can say some scary stuff though). Have a look at AIMS for some resources

I had this where I went for the 12 week scan and was told I was 13 weeks (not possible based on ovulation dates). Not a problem at first but when I got to 40 weeks the hospital booked me an induction. I arranged a meeting with a senior midwife at 40 weeks and came out of the meeting officially 39 weeks again! DP and I are are both 3-4 inches taller than average and our kid is too, so I think that could have been a factor in the measurement.

Just paranoid that if baby keeps getting bigger and bigger, by the time I'm 38 weeks, they'll measure me at 40 weeks

I’ve been told that they don’t keep remeasuring, as 12 weeks is the most standard point in development and after that, it varies again. So at 38 weeks, you’ll be 38 weeks; even if you’re measuring bigger.

You will get a say in what happens in later pregnancy, too. You’ll have a better idea of baby’s growth then, so you’ll likely know if baby is just bigger!

I have a close friend whose last baby was big, much bigger than her first 3. She wanted to hold out for 42 weeks - she had extra scans to check on the baby but she was allowed to, the midwife was supportive. In the end she went into labour at 41+4. He was a chunky baby, but absolutely fine

I stressed about all this stuff too, but it seems to be fairly straightforward and work quite well. Although it’s very weird to me that birth isn’t really talked about medically until the 3rd trimester - I guess there’s a long way to go until then but I didn’t expect it!

I knew exactly when I ovulated with this current pregnancy, and ovulation was very clear on my chart so no doubt at all. At my 12 week scan they added 2 days. I don't think such a small amount matters. If it was more like a week, I'd have queried it, purely because you don't want them to think you're 42 weeks when you're actually 41 - induction ahoy! But a couple of days is fine. Babies are all different; you wouldn't expect them all to be absolutely identically sized at 12 weeks to the nearest millimetre :)

The foetus is like 6mm. A day of difference is barely perceptible. They will tell you a EDD at your 12 week scan. There is literally no point in thinking about it before that

You'll only be dated officially at your 12 week scan. If you're measuring bigger later on in your pregnancy you'll be offered induction based on the size of your baby not any new dates.

Don’t over think it op. You’ll get your date at 12 week scan and that’ll be your final date.
It’s very common for a day or two difference.
I had two private scans and a day was added onto both, 12 week scan I was giving measurement/date the same as previous scan.

Ignore measurement, just the same date was giving

Thanks all! All very reassuring! Have my "12 week scan" a week on Tuesday when I'll be 12+5 (or 13+1 based on yesterday's scan). Very exciting 😊

Others have already answered but just to reassure you it's completely normal. I also temp and use okps so knew exactly when I ovulated but at dating scan at 12 weeks my due date was brought forward by 1 week. Funnily enough that was nearly a week ago and baby still not here