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How accurate is the due date the doctor gives you when baby is scanned at first appointment? Really confused about when I'm due!

(47 Posts)
Pinkflipflop Wed 08-Aug-12 21:46:18

I have thought I was 12 weeks pregnant but when the doctor scanned and measured the baby yesterday she said I was about 14w 5 d. I have very long and irregular cycles, so it's hard for me to be totally sure when I actually got pregnant. However if I could back 14 weeks and 5 days it was before my last period confused

How will I be able to find out for sure?

BonkeyHasGOLDMollocks Wed 08-Aug-12 21:51:03

I would go with what the doctor said.

They know what they are talking about. They got my dates perfect. (I knew the dates, they matched them) . Its amazing really.

I see your confusion about your period though.

Look at it this way, you have to wait two weeks less to meet your baby smile

Pinkflipflop Wed 08-Aug-12 21:52:56

I understand what you mean but I just don't understand how I can have my period after I got pregnant!!

MB34 Wed 08-Aug-12 21:53:56

I'm in the same situation, however, I bought a book (by Dr Mirian Stoppard - I think) and it said that pregnancy is only for 38 weeks and doctors say 40 weeks (from roughly the first day of your last period) as an estimate to cover the 2 weeks you possibly weren't pregnant.

It is confusing and seems all just guess work to me lol!

JollyHockeyStick Wed 08-Aug-12 21:54:17

It could have been implantation bleeding rather than a real period?

I thought I was 12 weeks pregnant at my first scan but in reality was only 10+3. This was the gestation they used to predict my due date.

RandomMess Wed 08-Aug-12 21:54:56

You can have a period when you are already pregnant.

Early scans are accurate within about 3 days.

JollyHockeyStick Wed 08-Aug-12 21:55:23

And yes, 14w 5 days is only 12 weeks and 5 days since ovulation, so please ignore the first part of my last post.

BonkeyHasGOLDMollocks Wed 08-Aug-12 21:55:56

Some people have periods all the way through.
I had a light period before I knew I was pg, turns out it was withdrawal from the pill.

Maybe it was early bleeding?
I had a bleed at 5 weeks. All fine, just one of those things.

Did you mention it to your doc?

maybe at your 20week scan you could ask them again if they are sure. May be easier then.

RancerDoo Wed 08-Aug-12 21:57:07

How much before your last period? Was the 12 weeks back to the first day of your last period?
Early scans can be out - mine was by six days (I know very precisely when I ovulate).

neolara Wed 08-Aug-12 21:58:35

I had a 12 week scan that was a week out. I'd had surgery and had only done the deed once that month. I knew with absolute certainty that my dates were right and their dates were wrong.

propercharlie Wed 08-Aug-12 21:59:06

Because you are not pregnant for the first 2 weeks!!!!!!! So you need to take off two weeks from the 14+5 to get conception date. So you conceived 12+5 ago (AFTER your period) but your pregnancy gestation is 14+5.


nancerama Wed 08-Aug-12 22:00:18

Based upon the fact DH and I weren't even in the same country on the date that my scan pinpointed, I can categorically state they are not as accurate as the sonographers would have you believe. My date was about 5-7 days out.

I went very overdue and when DH was finally whipped out, several midwives and doctors said that they didn't think he looked overdue and my scan dates must have been wrong.

hhhhhhh Wed 08-Aug-12 22:00:50

Message withdrawn at poster's request.

propercharlie Wed 08-Aug-12 22:08:57

14 weeks pregnant DOES NOT mean you conceived 14 weeks ago. It means you conceived 12 WEEKS ago.

Pinkflipflop Wed 08-Aug-12 22:15:42

I'm sorry (I'm normally quite clever, honestly) but I don't understand how 14 weeks pregnant doesn't mean that I am 14 weeks pregnant blush blush

Normally it's the children in my Y4 class saying "I don't get it miss!!"

simpson Wed 08-Aug-12 22:20:12

I don't think they are hugely accurate either as when I had my first scan with DD I thought I was 12 weeks and I was told I was something like 10 weeks 3 days.

But like somebody else said my H was out of the country when DD would have been conceived.

She came on the due date given but was obviously over due and the midwife said the placenta was in a bad way (not alarmingly bad but obv. Nearly 2 weeks over due) and DD was over wrinkled etc with split skin.

She is now 4 and absolutely tiny so I wonder if she was just small in the scans too iyswim.

propercharlie Wed 08-Aug-12 22:21:14

It's because at the very beginning they ask you when you the first say of your last period was. It's a way of getting a fairly reliable date out of everyone. If they asked you when you conceived you may not have a clue!!

Obviously your period would have been roughly 2 weeks before ovulation/conception hence this strange non-pregnant two weeks!

If you are 14+5, you conceived 12+5 weeks ago smile

JollyHockeyStick Wed 08-Aug-12 22:39:58

Although they say pregnancy is 40 weeks, it's not. It's actually 38 weeks from conception to birth. So in order to make it seem like 40 weeks is the average pregnancy they add 2 weeks to the length of time you have actually been pregnant.

There's really no point in trying to work out why they do this, it's best to just accept that some medical professionals are slightly bonkers and this is the way that they decided to do it.

soandsosmum Thu 09-Aug-12 06:48:05

I wouldn't worry as they update your edd at your 20 week scan. My 12 wk date was over a week out both times (i knew the day of conception both times due to assisted conception and only single episode sex!) But the 20 week date was only a few days out.

20 weeks probably seems like a long way off now, but its still only half way so plenty of time to plan and prep

IfElephantsWoreTrousers Thu 09-Aug-12 06:59:05

Like MB34 said - the official "weeks pregnant" number always refers to the-date-of-first-day-of-last-period-assuming-a-28-day-cycle so every pregnancy includes circa 2 weeks of not actually being pregnant - basically as far as medical science is concerned you a 2w+1d pregnant the day after conception.

I assume this is because until ultrasound etc was invented, doctors only had first-day-of-last-period to measure from, and you wouldn't want to have to rewrite all the text books changing the "weeks" numbers as there would be terrible confusion - best to keep the old nomenclature.

However, your baby will come when he/she is ready - remember that in France "full term" is considered 42 weeks not 40 - due dates should only be considered a rough guide, not a schedule!

PeshwariNaan Thu 09-Aug-12 08:23:05

Baby can grow past expectations. I know someone whose dates were exactly what she'd thought at an early scan. By the 12 week scan they were telling her she's due two weeks earlier than what they'd previously said. The same thing happened with her previous baby!

greenwichgroove Thu 09-Aug-12 08:31:46

I had this confusion with dates with dd1...
She was norn apparently a month early but when she was born she appeared to be full term development wise and was 7 lb 3.

greenwichgroove Thu 09-Aug-12 08:32:41

Born not norn

nannyl Thu 09-Aug-12 08:36:42

Mine was out by 3 days

I knew when i ovulated... i had charted / Fertlilty awareness etc, used OPKs and a CBFM,

when i told the sonographer that i knew that (this was around 12 weeks) I was told "these scans are only accurate to within 7 days" and we go by these scans

and yes if you are 14 weeks pg it means 14 weeks from period not from conceieving

CaseyShraeger Thu 09-Aug-12 08:38:02

Think back to before they had scans. The only thing they had to go on then in estimating due date was the date of your last period. So they established that babies generally arrived 40 weeks after the start of your last period (actually I suspect they noticed that babies generally arrived 41 weeks after the start of your last period but thought 40 was a nicer round number), and the whole 40 week pregnancy principle was established on that basis. Of that 40 weeks you are only actually pregnant for 38 (you don't conceive until the day that's counted as 2w0d pregnant), but that method of counting became established. Now there are dating scans and so forth so date of last period isn't the only thing to go on any more, but we are stuck with that way of calculating. To get an approximate CONCEPTION date you need to take 2 weeks off your "how many weeks pregnant are you" number.

Some places you will see both figures given - the pregnancy figure based on 40 week reckoning and a "gestational age" figure based on how long since conception. But if anything ever doesn't soecify then it will be the 40-week, "pregnant"-for-2-weeks-before-you-conceived, system.

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