Dating scan accuracy

[Amycoll02]

How accurate are dating scans. Had mine today thinking I'm 12 weeks on the dot. I logged my last 4 periods and also did ovulation tests daily to keep track. I went for my dating scan today and the sonographer said baby is measuring 11wk3days. She asked if I was on the pill and if I was regular. Not on the pill and yes a regular 35 day cycle. She seemed a bit baffled. My husband works away and the last time we had sex was 4 days prior to ovulation according to my tests. Then he was away for over a week and we didn't have sex very soon after he was back either. I mean I'm delighted to be pregnant but seemingly can't do the math. I was breastfeeding my 9 month old at the time and mentioned that towards the end and she said that makes sense but I still don't really understand how when I know my last period, my cycles and know when I ovulated. I feel I'm thinking way too much into it but this pregnancy has my hormones all over the place 🙃 also she labelled my uterus as uterus large and this is my 5th pregnancy and I've never seen that before?? thanks for any insight 😊

Pic of my little baba ❤️

They’re not 100%, but if you have regular 35 day cycles this sounds pretty accurate. What day of your cycle did you ovulate? CD20, CD21? if so then, a standard pregnancy assumes 28 day cycles with ovulation on or around CD14/CD15, so you would be 5-7 days later than the “standard” causing you to be 1 week less pregnant than your period date would suggest. With people with longer or irregular cycles, your “start of your last period date” is more accurate if you go back 14 days from when you ovulated.

However, there are so many other variables, it’s why it’s called an estimated due date, including implantation time, the scans being off by a millimetre or two. I wouldn’t over think it.

I wouldn't worry at all, everyone I know has had their estimated due date moved by at least a few days at the dating scan, and babies grow at different rates. And it's estimate anyway. As PP said, with a 35 day cycle, it makes sense you are not as far along as you think