How reliable is due date at 12 weeks scan?

[Savama]

At the start of my pregnancy I had 2 scans, one at 6 weeks and an other at 9 and they were both consistent with my dates, but when I went for my 12 weeks scan they said baby was a week older. It didn’t bother me at the time, but now because of partial placenta previa I need to book a c section and I been wondering about it...if I get a c section at 37 weeks, it would mean 36 weeks if my dates are right.
Sorry for the long text, but just wondering about this due date given at 12 weeks scan...

My due date was estimated the 24th at the first midwife appointment, 12 week scan was changed to 22nd and he was born on the 17th.
Not sure if that helps. But dates change all the time.

Due date can change at every scan. Its just an estimate date. The consultant should look at the last due date that was provided by the sonagrapher. This must be a scary and anxious time for you. If u have any questions ask your consultant or midwife. They are used to it and should be willing to discuss it further to out your mind at ease. I hope u all the best.

My early scans were out by a week or two. 12 week scan is more accurate.

Well it should be the most accurate scan as that is why they are scheduled for as close to 12 weeks as possible. 37 weeks is very early for a c section so I assume that they will be taking precautions in terms of steroids anyway to help baby. Mention it to your consultant if you are worried.

6 and 9 weeks is far far to early to give a real representation of your due date. It's just far to small and so is magnified on the ultrasound images making it harder to measure accurately. 12 weeks is more accurate. 1 week out isn't that uncommon. Your due date is worked out by your midwife from the first day of your last period anyway which won't be the day you got pregnant. It will be around the time you ovulate so 1 week out is more realistic.

It is currently believed that the best estimate of gestational age is made from a scan at about 12 weeks. However, it can only ever be an estimate & most babies are mature and are naturally born between 37 and 42 weeks. Whenever possible c/sections are not scheduled as early as 37 weeks, but when potentially very major issues such as placenta previa pose a risk to mum and baby there may be a good justification to deliver earlier. I assume you are being seen by a consultant. It will be for them to advise on the timing for c/section and I would expect them to discuss the decision and advice with you, including discussing gestational age at delivery and the possible use of steroids to help prepare baby’s lungs

Hi all, thanks so much for replying to me.
I’m going to talk to the consultant soon and see what he says, but it seems to me that if between the due date by last period and 12 weeks scan the difference isn’t greater than 7 days, they can take the first one as a guide. If it is greater than 7 days they would rely on the 12 weeks scan.
Let’s see...

@Savama They base the EDD on the assumption that the foetus is on the 50th centile. Clearly not all can be, some will be larger than that and some smaller. It will be more accurate than using LMP which has its own assumptions (average cycle length of 28 days with ovulation at day 14), but if you know for certain when ovulation did occur (OPKs + BBT confirmation, or IVF) then that is more accurate. I researched this extensively when my dating scan gave me an immaculate conception by at least 3 days 🙄. Incidentally at the dating scan, the sonographer said she would discount the most extreme measurement which put me an entire week ahead when I freaked out about it (which clearly makes the entire thing a bit dodgy). The scan we had two weeks prior at the Harmony test agreed with my dates to within one day. It took me a while but I got the EDD changed to reflect my data.

I'm currently booked for ELCS at around 37 weeks as my waters have been trickling since 34+4. I'm on prophylactic antibiotics but they aren't bothered about steroids at this stage (I think they said the cut off was pre-34 weeks).

It's not an exact science, at my 12 week scan the sonographer measured twice and the first time the baby was 12+1 and the second time she was 12+4. Based on what I know (I was tracking ovulation) I think the first one was close to being correct but she went with the latter so that's now my official due date.

@Weathergirl1, I saw this thing about 50 centile and the variations on the internet, but when I talked to the sonographer who is also a doctor she was so strong about it saying all babies at 12 weeks measure exactly the same therefore she wouldn’t change my due date.
I was tracking ovulation, done it for 4 months before getting pregnant. Done the early scans...

@Switcher33 definitely not a exact science, just annoying that might I need to book a c section with baby being younger than they say she is.
I also don’t know what to think about the steroids injections...seems very common this days, but I saw some scientific articles saying things against it unless it is the only way like it is for babies at risk of being born before 28 weeks.

With DS2 they moved me forward a week at my 12 weeks scan. I knew when we'd had sex and it meant my due date wasn't physically possible.

Once I got to my planned c section I waited til 38+6 as I knew it was closer to 38 weeks.

@savama your doctor was telling porkies... I didn't just Google, I looked at the medical literature! They don't find it convenient when they get people questioning the normal practice. It took me ages to get my date changed, but the head of midwifery I spoke to a few weeks ago (about something else) didn't have an issue with it. It's really up to you, but for me, as someone with general anxiety disorder (especially around medical issues) I didn't want to be having to argue over it towards the end if they were going to try to make clinical decisions around an EDD that was wrong.

@Weathergirl1 I agree with you, some doctors really don’t like the questions, I could see the annoyance in the body language of the assistant as well.
Fingers crossed the appointment with the consultant will be a different story, otherwise I’m even thinking of going privately... I’m not rich or anything, but I don’t want to feel like something so important and people aren’t willing to listen to you, it is a horrible feeling.
So fingers crossed the consultant will listen and give me some good explanation on everything.

@savama it's annoying isn't it, I've had appointments where I've felt that the midwife/doctor wasn't listening to me and was just dismissing my concerns and it is so frustrating.

I'm not a medic but I'm surprised you were told that all babies measure exactly the same at 12 weeks, that doesn't sound right. Even if that were the case, I've always heard that on ultrasound it's impossible to be 100% sure of measurements as the baby's position or even the sonographer can make a big difference and just a few mm will push you up or down the centile quite significantly.

For your appointment, maybe write down all your questions in advance and print off any research papers you've read. May just be me but I always find I get taken more seriously when I do this and they spend more time explaining things properly. Also remember you can always request a second opinion.