Incorrect due date at 12 week scan?

[Keha]

Hiya, I'm 25 ish weeks pregnant. When I had my 12 week dating scan we were there for about 2 hours and the sonographer had to try several times to get measurements. When they had finished they gave me a due date which I think is a week earlier than is possible. I think this because I was tracking ovulation and took pregnancy tests optimistically early. If I'd have conceived in line with the date given I'd have got a negative pregnancy test after my period was late, which doesn't add up. I have told my midwife. My worry really is that my last baby was big and I am being sent for a growth scan. There has already been some talk of induction. I don't want the dates for this to be based on an "incorrect" due date. Anyone have any experience of anything similar?

Yes, my due date was brought forward by 6 days at the 12-week scan, due to the baby's measurements. I know exactly when I could and could not have got pregnant in that cycle, and I'm not prepared to accept a comparison with population-level data over the dates I know to be possible. I've made clear to everyone I see that if I go 'overdue' I will only discuss induction based on my original dates, not the scan estimate.

The NHS 12 week dating scan put me 6 days a head and I was so sure when I ovulated due to using OPKS.

Went into Labour at 40+6 days. So 6 days late by NHS but smack bang on time by mine!

The dating scan is only an estimate based on the measurements I guess so won't be 100% but they will go by their dates!

If the growth scans track and there's no other worries, just say no until/unless you do want to be induced. Mentally translate any risks to your corresponding date to decide what to do.

Thanks for your responses, nice to know I am not the only person this is happening to. I am quite confident to say what I think and refuse what might feel like an overly early induction (especially as I have had a spontaneous labour at 40+2 with my first (huge) baby). I am a bit worried though that the Drs are going to try and push me into it/make me feel like I am taking some terrible risks when honestly I just think the sonographer struggled with the measurements. I've already been booked in for a growth scan and consultant appointment at 34 weeks based on the size of my previous baby.

I've recently had this at my 12 week scan, they’ve said I was 12 weeks 3 days which is impossible as it would mean my last period was 12/07 not 15/07 (I know it was the 15/07)! Should I be worried?

Very common for dates to change either way by a few days at 12w scan based on babies growth

@Jxx 3 days in the grand scheme of things won’t worry too much! If it gets to induction talked and 40 weeks + 14 you can explain your situation.

I would review after your late growth scan OP. If baby is big you can discuss your options and discuss your concern with dates then. No point stressing until you know!

Thank you for responding! It just doesn’t correspond with my period in July and my period in June was 14/06 so if I’d got pregnant from that month I would have been 16 weeks at the week scan not 12 3 days, the dates just don’t add up at all and it is very baffling! X

Yes. Was brought forward by 2 weeks. DC arrived on original due date. I wouldn't worry too much about it. They are keeping an eye on you and the main thing is if anything changes that you are worried about that you tell them ASAP. None of this is a precise science and ultimately they go with what Mum says (or that is the theory anyway).

Haha you’d like to think they do, I actually tried telling them the date I ovulated (as I tracked) and they were having none of it, still insisted I was earlier than I possibly could be xx

@Jxx there's loads of reasons dates can change slightly, and as others have said 3 days isn't much in the grand scheme of things.

I know you tracked ovulation, but the OPKs basically show when you're roughly 12-36 hours before ovulation, so there's a day's leeway there. Then implantation can vary from about 6dpo to 12 dpo (ish) so if your due date perfectly lined up with your LMP you probably implanted somewhere bang in the middle of that window. A day or so earlier or later and it's going to move your dates a bit. So even if two people had a positive OPK on the same day, it's easy to see how their due dates could be a few days apart.

It's really normal to have dates move at the 12 week scan, it happens to most people by a few days at least!

I have had this, scan at 8 weeks totally lined up with my ovulation/ period dates and then at 12 weeks was suddenly put forward 1week 5 days to an impossible conception date. Not a worry in itself BUT I am now 34 weeks and fundal height measuring 4 weeks ahead of the 12W scan dates, so 5w5d ahead of my dates - potentially meaning a very giant baby!
I have a growth scan at 36 weeks to double check and talk to consultant but dont know whether I should point this out when I meet with them, as it makes the baby even bigger than they will be thinking based on scan dates?

Exactly same as me, I had a scan at 5 weeks perfectly lined up and again at 8 weeks. It was my 12 week scan that threw me completely and they didn’t explain it either! I had been trying for seven years so I’m on the ball with my periods / ovulation etc which I tried to tell them but they didn’t listen ha!
I have my 20 week scan tomorrow so nervous in case they change my date again! X

They even change ivf dates!
dc2 was largish so brought forwards maybe 4 days. Otoh when i got to almost 10days over - their date i wasnt as concerned dc would be bi g and need inducing.

@Keha can I ask how big your baby was the first time? I ended up with an emcs and had a nearly 11lb baby at 41+3 on their scan date or 40+3 on my dates. I'm not at the point of my 12 week scan yet but they're already suggesting I might get a growth scan this time and that I will probably need a section again 🙃

@Catson my first was 10 lb 4, my second (the one this post is about!) was 9lb 11. I actually went into labour on same gestation (40+2) with both. As pregnancy went on I decided not to worry too much about the due dates. Think I might make quite big babies.