Scan 10-11 days off - very confused

[renimummy]

Hi everyone... it’s my first time here and also my first pregnancy :)
I’m looking for some advice! I’m very confused about how many weeks I’m pregnant and would love to hear your opinion

So ... my last period was 3rd august which would make me currently 17weeks. However according to my last two scan I’m only 15 + 3 hence about 10 days off. I used to have irregular cycle anywhere from 23 to 27 days long. I travel a lot and that’s why I know exactly when in August we had sex wish my loved one. It was on 19th and 20th August. When I asked my doctor he said it’s all fine because I ovulated 10 days later than normal but I told him it’s not possible as there was no sperm to do the magic at that date :))) and also I understand sperm can live for few days but not 10... right?

So why do they keep moving my due date to 21.5 instead of 10.5? Is it because my little baby is simply too small? I keep reading that scans are not 100% accurate but isn’t 10-11 days too much?

Thank you so much ladies for your help :)

[mindutopia]

Sperm can live for up to 7 dates, and then even if you know your exact ovulation date, baby can still be up to about 5 days off from that date just due to differences in speed of development, so that doesn't sound too unreasonable.

With my first, I absolutely knew the exact day I ovulated and I had perfectly regular cycles anyway (but was also tracking ovulatation). But at dating scan, she was dated 4 days ahead. We had sex the day before ovulation, but had we had sex 7 days before ovulation and the sperm survived all that time, then my dates would have been 11 days off from the day we had sex. So it's absolutely possible. Dating scans aren't an exact science either, so a tiny bit of measurement error can mean baby is dated a few days earlier or later too.

As long as baby looks healthy, good heartbeat, all is well, it sounds fine to me.

[renimummy]

So far everything seems to be perfect with baby so I’m happy :)
So basically even thought I know exactly when we had sex that doesn’t mean that’s the day of my conception as sperm can live for up to 7 days? And then I guess 3-4 days off as scans are not super accurate so makes sense I guess.
Can they change my due date again at 20 week scan?

[spugzbunny]

I know the exact date I ovulated because I was checking my temperature to confirm. My 12 week scan put me a week ahead of that and I've measured consistently at that point. They will only change your due date if it varies significantly. At the end of the day, baby will come when baby is ready!

[sah2241]

I know exactly when I ovulated, and my date more or less matched the date I was given at my 12 week scan. However, at my 20 week scan they moved my due date back by a week. I queried this and was told that scans tends to be accurate to within approx. 10 days so the new date you've been given seems to be within the normal margin of error. And yes, they may still change the date at your 20 week scan!

[CL1982]

Maybe your baby just took a little while longer to implant and get going? It's very unlikely the sperm survived more than 3-5 days (that 7 days thing is very rare) BUT if you factor late implantation into that plus a slow growth (nothing wrong with that - mine did the opposite and grew like a weed instead the first few weeks) you might wind up 10 days behind :) I know when i ovulated and i think it just implanted VERY quickly (I had a BFP at 7 days which i found pretty mad) and it had a good grow in the first 8 weeks! So i'm 5 days ahead if that makes sense. Everything else has been bang on average.

They will keep your DD now OP as they've set it but maybe make your midwife aware?