Accuracy of dating scan at 15 weeks?

[Flisspaps]

Does anyone have any stats/info as to how accurate dating scans are after about 15/16 weeks? I've read somewhere that they're not particularly reliable at all after about 14 weeks but can't find anything to back this up.

I had a dating scan this week at what I thought was 15+4 and was very happy with this gestation and EDD.

The scan midwife (how she referred to herself, rather than as a sonographer) took three measurements which all came out as different gestations:

Femur 15+4
Abdomen (I think) 16+4
Head circumference 17+0

The midwife knew that I thought I was 15+4 and the computer only allowed her to enter one measurement to give the due date, and for some reason, she chose the head circumference. I didn't ask why as she was too busy quizzing me about things which were irrelevant to the scan, which I'd already discussed with another midwife at my booking in appointment, and forgot to ask.

That's a difference of 10 days, which at the moment isn't huge but it does mean that if I go post dates (by their new date) then I'm going to be put under pressure to induce on what I believe is my real due date (otherwise I'd have ovulated unusually early for me). This is a concern to me as I went to 42+2 with DD1 (not that I'll take induction this time).

Have an antenatal on Monday so will discuss this with the MW, but I'd like to arm myself with as much info as possible as to why the new due date might not be accurate.

I don't know how accurate they are at that stage, but I know the reason for having the dating scan at 12 weeks-ish is that up until that point roughly, all babies grow at a very similar rate so the measurements give an accurate date. After that, they all grow differently so its not as accurate.

However there are a couple of things to remember 1) babies very rarely come on time anyway, so it's only really an indication and 2) they can't force you into an induction, although they may strongly recommend it if they believe that there is a really good medical reason for it, and not being induced would be a risk to mum/baby. They will probably just monitor you closely if you refuse, just to make sure. Good luck!