Estimated due date

[ohgoonthen]

Hi everyone,

I have a question I hope someone can help me with. I'm pregnant with first DC. Baby wasn't planned (was on the pill at the time) and we didn't find out until I was about 6 weeks.

Obviously, I don't know the exact date we conceived as we weren't trying and I can't remember the exact date of my last period (I never went by dates as I have been on the pill for years). I think it was around 15 Dec.

So, when we went for our first scan, babys due date was 23rd September and at all our scans, it has been around the same give or take a day or two.

When we went for a scan yesterday, the scan said the due date is now 12th September.

Can anyone explain why there is such a difference? I didn't get to ask the Doc as he was called for an emergency and I know it's not a huge deal, but It has been niggling away at me.

Thanks for all your help

Hi, I'm expecting my first and dates have changed for me too, I think it goes off the measurements of the baby as they measure around the head etc... I could be wrong but this is the impression I got. Don't let it niggle, the baby will arrive when he/she wants to anyway! congratulations and if it helps my last period was 10 jan (i'm exact as we were trying) and my EED is 22 Oct so you'd be about right

Here they go off your LMP until the 12 week scan, which is a dating scan, once that scan is done they use that date as EDD.
My EDD is at the top of my hand held notes and wasn't written on there until 12 weeks scan (by midwife), and that is the only date they are using for me.
HTH.

It's odd if they have changed your due date based on the 20 week scan - normally the 12 week scan (and any scans before that) is considered to be a lot more accurate. I would ask your midwife why they've done that.

I'm totally confused myself. I'm not in the UK so I don't have a midwife, the Doctor here does everything.

When I first went, he took details of LMP and scanned me, the scanner put me at 6 weeks and 1 day which I thought would have been about right. Twelve week scan changed the due date by 2 days I think and 20 week scan (which I had 2 weeks ago) says Gestational Age (from dates) 23 weeks + 1 Day, EDD: 21/09/10.

So, all dates have seemed to agree so far apart from yesterdays. As I said, the doctor was in a hurry and only got to measure the babys head, so I reckon either he has made a mistake or baby is quite big. He says that her estimated weight according to the scanner at the moment is 1kg, but I'm not sure if that is big or small at this stage!

I think it works like this in the UK (I'm on my first preganancy so please could anyone who knows better please correct me).

1. Calculate due date from LMP.
2. Have 12 week scan and a due date is calculated from this. 12 week scan date is generally the agreed due date. Big differences might be investigated but I have no experience of this.

1. Later scans - measurements are taken to see if the baby is growing normally. Mostly you only have a 20 week scan and then subsequently the fundal height is used to check growth. If significant differences are noted in the fundal height a scan might be used to take more accurate measurements.

The 12 week scan is used to determine the agreed due date as most babies are the same size at that stage.

Gradually babies grow at different rates but provided these are within the normal range then this is fine and just results in a smaller or larger end result. They don't change the agreed due date just because your baby is growing slightly more and perhaps will end up on 80th centile at birth.

In our hand held notes we have a graph which shows how the baby's head circumferance should grow and has a shaded area either side of the average line which shows what is considered to be the "normal" range.

If you look at this page you can see that if a baby has a head circumference of 240mm then it might be anywhere between 24+1 and 27+5.

Hope this makes sense.