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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.

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.


AnythingNotEverything Fri 03-May-13 13:58:08

Also, conception doesn't necessarily happen on the day you dtd!

serin Fri 03-May-13 13:59:58

grin it will come when its ready!

TeamEdward Fri 03-May-13 14:02:50

Due to my long cycles, I had worked out a due date 12 days later than the official LMP due date. Despite gentle protestations that my LMP was not a reliable indicator, the due date was never changed.
DS1 was "officially" 12 days late...

kilmuir Fri 03-May-13 14:03:14

If you go 2 weeks over you could be in hospital opening your Xmas pressies

ShowOfHands Fri 03-May-13 14:06:33

Your due date is largely meaningless in terms of when your baby will arrive. It's just an aid to appropriate care. A normal pregnancy lasts between 37 and 42 weeks so you have a large window really. And a Christmas baby is a bit special. The calculation from the last period gives a rough, inaccurate guide, first scan gives a much better guide and then the baby comes when it wants anyway. grin

Have you got any decent pregnancy books?

HoneyDragon Fri 03-May-13 14:13:33

Yup. On both my conception "dates" my dh was out the country grin <<hussy>>

Fenaklu Fri 03-May-13 14:20:59

Thanks people (esp midori1999 and mrsden) - for some reason I was thinking that the due date was calculated from fertilisation of the egg rather than implantation which is why it didn't make sense that the known date of sex was ignored - I always thought they calculated from the last period only if they didn't have a reliable date for conception!
And for those asking why I didn't query it at the time - it was only just now I realised that the dates didn't seem to add up - my mind was going round the SH*T I'M PREGNANT hamster wheel at the time!

knittingirl Fri 03-May-13 14:51:13

The thing is, even if you know which act of sex made the baby, you still don't know when fertilization occurred. Sperm can hang around in the uterus/fallopian tubes for up to five days, and the egg lives for about 24 hours from ovulation. So you could have sex at any point within a six day period, and still "conceive" on the same day, if that makes sense.

Hence, they go with lmp as an approximation, then date you properly at the 12 week scan.

Startail Fri 03-May-13 14:59:40

They still try to go from date of last period even if you've had IVF and know precisely when the baby was conceived.

Drove DF nuts trying to explain that 38 weeks after conception was a more accurate date than 40 weeks after her last period.

MummyBurrows Fri 03-May-13 16:03:10

First off- Congratulations!! [Grin]

Secondly- I really wouldn't worry about when the doctor says your due. The due date will change with every scan you have anyway! FWIW I know full well I didn't concieve my current pregnancy anywhere near the time my gp said,I also know that I didn't concieve on the dates either my early dating scan and 12wk scan gave but the weeks thing is just to make it easier for scans and midwife check ups and telling people when they ask "how far gone are you?" really,they're very rarely accurate. We all know that most babies arrive anytime between the 38-42wk mark on average and that "due dates" are nothing but a rough guess meaning "sometime around that date but we don't actually know,it could be a week or 2 either way"...baby will come when he or she is ready so don't obsess too much over any due date you get given,just make sure you're ready with a hospital bag by the door,I personally have mine ready by the time I'm 30wks-just incase!xx

Gonnabmummy Sat 04-May-13 15:01:11

Pregnancy is dated 40 weeks from first day of last period it's most accurate way albeit 2 weeks either side anyhow. Most women know for definite when last period was and conception can happen at diff stages

Applepiesky Sat 04-May-13 16:50:59

Think a few of you are being rather harsh on this lady. She has already explained she is a first timer, as am I, and although I knew how due date is worked out, there's a HELL of a lot I don't know. Hence the reason for these forums, for people to be able to ask questions, get help and advice. No need to accuse her of being accusatory when she is just looking for reassurance! Have noticed this a lot on here, people can be so quick to judge and often respond to some people most negatively!

Bodicea Sat 04-May-13 18:34:30

Also you never know for sure date of conception, sperm can survive up to four days in body so can conceive much later than when you had sex. That's why lmp is more accurate than conception date x

Christelle2207 Sat 04-May-13 22:00:39

OP just to make it clear that in the UK pregnancies are calculated as 40 weeks long despite the fact that there are only 38 weeks between conception and due date - i.e. you spend the first two weeks of your pregnancy NOT pregnant. This is based on average women conceiving 2 weeks after the first day of their last period but up to a week either side of this will be common.
As others have said your scan will give you a more precise due date.

Christelle2207 Sat 04-May-13 22:03:20

ps day of implantation, typically a week or so after conception, doesn't come into it at all. As others mentioned you could have ovulated several days before you conceived.

pinkteapot Sat 04-May-13 22:14:48

Apple, its always ok to ask questions and wise to try find things out. I think what I found a bit funny was the phrasing of the question, mostly the assumption that the gp had "deliberately" given wrong due dates, as opposed to mistakenly done so or simply done something the OP hadnt understood (which is as it turns out). just sounded like the OP had made her mind up gp was in the wrong.
.. Anyway, mountain out of a molehill at this stage.

WeOnlyPlannedTheFirst Sat 04-May-13 22:16:31

I read somewhere that in Europe (maybe France) it is usual to think of a due month rather than a due date. I can see how that helps stop people getting hung up on a specific date and all the people hassling for news of twinges when you're just one day 'overdue'.
Regardless of the date you are given your baby is going to come when it wants to. Hopefully not too early but if your due date is 10 days before Christmas then a Christmas baby is a possibility. If your due date is middle of January a Christmas baby is still a possibility!
OP - Congrats on your pregnancy and enjoy it. I'd suggest getting a good pregnancy week by week book - they're very educational plus it's fascinating learning how your baby develops over the pregnancy. It was my regular Sunday treat when expecting my first to sit and learn he was the size of a plum etc!

Booboostoo Sat 04-May-13 23:04:32

No, in France they do dating scans and tell you a date of conception from which they calculate. In Greece they go by date of implantation a week later which can cause confusion (well it did for me going between the two countries!!!).

Congratulations OP!

Bodicea Sat 04-May-13 23:26:28

In most hospitals they take LMP date over date from scan unless there is more than 7 days difference or pt doesn't know LMP date. I am a sonographer and it annoys the hell out of me that I go to the trouble of getting accurate measurement to give accurate date and then they ( that is that powers that be) choose to ignore it!! angry

Quilty Sun 05-May-13 09:30:35

Even if you know the exact date of conception, dates are still calculated from an estimated LMP. Even for IVF couples, the date is still calculated as though it was a natural conception so that the measurements from the dating, anomaly and growth scans fit in with the charts. If we didn't do this then measurements would appear to be underestimated by around 2 weeks.

As others have said I find it quite bizarre that you would think the doctor had purposely given you the wrong dates! I think you need to try and not be so suspicious and trust that the people involved in your care actually know what they're doing!

Celaun Sun 05-May-13 09:40:11

That's the way pregnancies are dated - 40weeks from last period. It means it actually always includes approx 2 weeks when you wen't actually pregnant but that is the case for everyone. You will also get an estimated due date at your 12 week dating scan based on fetal measurements.

Featherbag Sun 05-May-13 09:45:38

Hi, welcome to the club! Have you introduced yourself in the December ante-natal thread yet? I'm due 7th December (based on LMP), but this is DC2 for me so I know that a) this will change with every scan and b) baby will turn up when it bloody well wants to anyway (at 32 weeks in my case!).

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