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Doctor gave a deliberately inaccurate due date - why?

(47 Posts)
Fenaklu Fri 03-May-13 13:26:21

I found out a couple of days ago that I was quite pregnant (more than 3 weeks on digital test). Last period was 7-9th March but due to illness on holiday, 14 hour flights and jetlag etc, the earliest possible conception date was 18th March.
My GP has just given me a due date of 11th December which makes me 8 weeks pregnant when I am only 6 and a half weeks (from known earliest conception date). This makes my real due date 10 days closer to Christmas - something we really needed to know!
Why the deliberately innacurate calculation???
(This is my first at 41, so he has no previous gestation data to go on)

OhBuggerandArse Fri 03-May-13 13:27:51

They don't date from conception, they date from first day of last period. Quite usual.


Mintyy Fri 03-May-13 13:29:46

Have a look at the due date calculator on Mumsnet. As OhBugger says, they date from the first day of your last period.

flowery Fri 03-May-13 13:30:01

NHS due date calculator with a first date of last period as 7 March and a cycle length of 28 days gives 12 December as due date. Is your cycle longer then? What makes you think he calculated then adjusted it to be wrong deliberately?

Slainte Fri 03-May-13 13:30:07

Due date is calculated from first day of your last period.

Congratulations smile

heidihole Fri 03-May-13 13:30:22

You have a very accusatory attitude! Do you normally believe the whole world is out to get you?

This is how the dating works. They date from last period so on date of conception you are already counted as 2 weeks pregnant (yes crazy but that's how they do it)

Slainte Fri 03-May-13 13:30:42

Massively x-posted blush

flowery Fri 03-May-13 13:33:23

It is a bit strange to assume the GP has deliberately put the wrong date, rather than either has made a mistake, or has not explained the calculation.

NoTeaForMe Fri 03-May-13 13:35:18

A little harsh heidihole , but I do find it funny that you immediately thought the dr deliberately dated you wrong rather than it just being the way it's done, OP!

Pregnancy is dated from the first day of your last period, as everyone has said!

FannyFifer Fri 03-May-13 13:36:37

Dated from last period not ovulation as everyone has said. A quick google or looking in a pregnancy book would tell you that.

RedPencils Fri 03-May-13 13:40:46

It's the way they work it out! They woul never be able to predict dates if they had to start going into when you ovulated, when you had sex. Most people don't know.
When you have a scan they will correct if necessary.

HoneyDragon Fri 03-May-13 13:44:49

In the uk pg is dated from lmp

In the some other countries the estimate date from when you last had sex.

And Christmas babies are AWESOME.

mackerella Fri 03-May-13 13:45:00

(Wrote a long post explaining how due dates are calculated, lost it and refreshed the page to find out that everyone else had said the same thing anyway!)

Are you in the UK, op? I only ask because gestation is counted differently ob some European countries - from (assumed) date of conception rather than last menstrual period (LMP). Under that system, you'd be deemed 6 weeks, not 8. However, gestation is also considered to be 2 weeks shorter (38 weeks rather than 40 as it is here), so you would still have the same due date. I only found this out after some rather confusing conversations with my SIL, who lives abroad!

Incidentally, if you are in the UK, you should be offer a dating scan around 12 weeks, which will give you a more accurate due date anyway.

megandraper Fri 03-May-13 13:46:49

it is a bit confusing, i am another 'older' mum and i was quite surprised when i found out, couldn't believe I'd never known that.

flowery Fri 03-May-13 13:50:35

If you thought he'd done it deliberately wrong, for some reason, why didn't you say there and then that you'd calculated a different date? He would no doubt have explained it to you.

Fenaklu Fri 03-May-13 13:51:16

Cheers guys! What i meant was, if someone knows the actual date of conception, why deliberately calculate it from 10 days earlier, knowing full well you have erroneously added a week and a half on (which is quite a lot in the early stages) I can understand dating from the last period if the conception date is unsure, but it seems stupid to do it in this case (my GP is a great guy - I know he isn't out to get me lol just wondered why it is done like that at all!)

pinkteapot Fri 03-May-13 13:51:48

afraid i'm with heidihole in thinking its a bit accusatory... if you thought it was 2 wks out OP, wouldnt hurt to just ask the gp why...? simple enough explanation as all the pps have said. as opposed to the gp deliberately doing his job wrong!

pinkteapot Fri 03-May-13 13:52:31

x posted - fair enough

givemeaclue Fri 03-May-13 13:54:23

Didn't ask?

Congratulations but you may wish to adopt a slightly different approach with the health professionals looking after you.

Lolapink Fri 03-May-13 13:54:44

,your due date is calculated from the first day of your last period.

NinjaChipmunk Fri 03-May-13 13:55:28

when you have your 12 week scan they may well amend your due date due to the baby's measurements too.

Fenaklu Fri 03-May-13 13:56:15

Thanks mackerella - I didn't know that they knocked 2 weeks off if they calculate from date of conception - it is reassuring to know I am less likely to be in hospital over Christmas than i feared - both my sister and sister-in-law had Christmas babies (24th and 27th Dec)!

midori1999 Fri 03-May-13 13:56:33

Because that is how gestation is calculated. They can't calculate it one way for some people and another way for others. Plus, you don't know the exact date of conception, because even if you know the date you had sex, you will not necessarily know the date you ovulated or implanted. If there is any difference from the date calculated by your last period, you will find out at your dating scan.

mrsden Fri 03-May-13 13:56:41

because that's how pregnancies are dated in the UK. There has to be a standard system in each country, it makes little difference what that system is so long as everyone is using the same. Ovulation is not a great one to use because most people don't know when they ovulate, and even knowing when you ovulated it's not possible to know exactly when conception happened. The vast majority will know the date of their last period so it gives a good starting point. There will be a revised date after the first scan if the size is out.

AnythingNotEverything Fri 03-May-13 13:57:10

Your due date is just a guide anyway - and is a guess really until you get some scans.

I read somewhere that only 7% of babies are born on their due date.

This is the first of many steps where you'll see that pregnancy, childbirth and midwifery are not exact sciences.


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