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Confused about the phrase 'weeks pregnant'

(19 Posts)
FeralGirlCambs Wed 05-Jan-11 15:45:44

Ok, I must be a complete duh-brain but I can't get my head round an apparently simple thing: I am pregnant and delighted. Last period started on 8th October, which puts me at 12-13 weeks pregnant by the conventional analysis - ie what the Dr, the midwife etc say. So I decide it's ok to tell a few people, and go in for my 12 week scan on New Year's Eve, whereupon the ultrasound person says baby's size indicates only 9 weeks, and I start thinking I've been really rash telling people. But there must be a distinction between 'weeks pregnant' and actual gestational age, i.e weeks after conception / implantation? After all, one gets pregnant not on day one of one's period. My cycle was really long, so date of conception prob about 3 weeks after 8th Oct (which first perfectly with the 9 weeks date). So my question is, isn't everyone going for a 12 week scan based on last period actually only about 10 weeks in reality? So do I say I'm 13 or 10 weeks? And why can't I go back for my nuchal scan till 12 weeks actual gestation (ie 3 weeks from the 12 week scan that was actually 9) - by then I'll be 15 weeks by the period measurement, they won't do it sooner - but surely most people who have it at 12 weeks are actually less than that in terms of gestational age...?? A stupid thing to worry about but it feels important! Any illumination much appreciated.

fel1x Wed 05-Jan-11 15:58:37

They use your last period to work out how pregnant you would be if you had an average 28 day cycle.
I do, so my last period dates and the scan dates will match up.
Your cycle is longer so they adjust for that once you've had your scan (which they did)
You are actually 9 weeks pregnant as thats what the scan says (and that overrides the previous estimate) iyswim

DilysPrice Wed 05-Jan-11 15:58:42

An average pregnancy is said (in the UK) to be 40 weeks long, measured from start of LMP, but for the average woman with 28 day cycle ovulating on day 14 she will conceive on week 2, and only actually be pregnant for 38 of those 40 weeks. We always ignore that when talking dates, hence " 12 week" scan will normally take place 10 weeks after conception.
However, if you ovulated significantly later than 2 weeks after LMP you will need to adjust for that, both to reassure yourself about your "small for dates" embryo and so that you don't let anybody induce you too soon at the other end.

tebbles Wed 05-Jan-11 16:06:11

Did they change the due date when you had the scan? This might make the difference.

AFAIK The due date is based on the first day of your last menstral period. This is based on the general assumption that you conceived 2 weeks after this.

If for some reason the baby's measurements do not corrsspond with this date at the scan then it means that you must have conceived at a differnt date (ie earlier or later) as your cycle does not follow the usual 2 week pattern.

AFAIK as they said you were 9 weeks on the 31/12 this makes you 9+5 (or 10 weeks this Friday) according to the medical world. I think the actual date of conception is considered irrelevant to them.

I wouldnt worry about not being 12 weeks yet as I think as long as the scan showed a healthy baby/ heartbeat the chance of miscarriage is much lower anyway.

ShowOfHands Wed 05-Jan-11 16:11:11

As others have said, pregnancy is 40 weeks on average. The 40 weeks starts 2 weeks before conception when the average woman will start her period. Obviously is you ovulate later that 14 days from your lmp then your pregnancy will not be as far along as somebody who has a shorter cycle and began their period on the same date as you.

You are generally as pregnant as the scan says you are.

FeralGirlCambs Wed 05-Jan-11 16:55:44

Thanks for all the responses! No, they didn't give a due date at the scan as I have to go back (and it was last appt on NY eve!) I'm still a bit confused though about whether the ultrasound lot are using the same definition of weeks pregnant as the conventional one - what Dilys says seems to make sense (ie 'normal' people are actually roughly ten weeks pregnant at the 12 week scan) My first due date was indeed 40 weeks from first day of period, minus two weeks.

What I'm getting at is I think there's only about a week difference between what the scan revealed and what I was originally told, and the other two weeks would affect everyone - ie everyone would be only ten weeks when they go for a 12 week scan? So felix's scan date and the date based on her regular 28 day cycle would still be two weeks different? So do the scanning lot tell you how pregnant you actually are (ie weeks the embryo has been growing for - surely that's all they can tell by measuring) whereas the conventional 'n weeks pregnant' measured from the period date includes those two weeks of nothing happening. If so, wouldn't it be easier if they used the same system (ie added the two weeks)?

Shouldn't really obsess about this. Partly I'm wondering if I'm three weeks further behind a friend who had a 12 week scan and did not have her due date changed than I thought I was, or only one week (which I guess I knew anyway, due to my irregular cycle, meaning I would conceive longer after my period than the average person)

Thanks for the reassurance tebbles - the small maggoty thing did indeed have a good strong heartbeat which was a fab thing to see for the New Year!

MrsSawyer Wed 05-Jan-11 16:56:15

Ooh me too, ive been wondering this, so my LMP started on the 28th Nov, my cycle is approx 38 days and we had sex on 12th and 18th dec and all the due date calenders tell me im 4 weeks pregnant, is that right?

My brain is completely fuzzled at the moment blush

ShowOfHands Wed 05-Jan-11 17:02:50

You're complicating things unnecessarily! They do use the same systems. A pregnancy is 40 weeks from lmp in the average woman. Or 38 weeks from conception if you like. The dates given at the scan are just more accurate but no different to the ones given by the doctors.

You go for your 12 weeks scan when you're 12 weeks pregnant. Yes you only conceived 10 weeks ago but they count the 2 weeks before that in when calculating.

All your scan is telling you is when you actually conceived. If they said you're 9 weeks, then you conceived 7 weeks ago. You just ovulated later than you thought.

FeralGirlCambs Wed 05-Jan-11 17:51:48

Oh dear, now I really am confused. So they are not telling me the embryo has actually been growing for 9 weeks? But I did a positive preg test 7 weeks ago (Nov 18th) so surely I can't have conceived then AND that would mean I didn't ovulate till 6 weeks after my period started on 8th Oct. My cycle was weird but not that weird.

Maybe this is a silly discussion to prolong as now I'm thinking the only answer must be they have got it wrong, which must mean baby is abnormally small and something disastrous is going to happen....

tlise Wed 05-Jan-11 18:03:57

Nothing is wrong or they would have said so stop fretting ;) normally they go from the date of your last period, but if they have said that its 9 weeks old, that means its 9 weeks old but should have changed your due date. when I went for mine, I was 12 weeks on the tuesday, but they measured it as 11+4 so put my date back 3 days so I was 12 weeks on the friday. Which meant I conceived 12 weeks ago on the friday.

It could be that they didn't even quite measure correctly. When I had all my others, if the date was less than a week difference, they didn't alter it as they said it allowed for human error. You are quite right about everyone going in early for the 12 week scans, I have thought that myself, but I suppose they know what they are doing..

Have you asked your midwife? She would be the best one to help, other than that you said you have to go back, so they might tell you more next time

missp2010 Wed 05-Jan-11 19:37:05

I have only read the replies really quickly, so sorry if I'm repeating what others have said. From what I understand a pregnancy should be 40 weeks, conception would be roughly 2 weeks after your lmp however they include those two weeks before because the egg was mature (or whatever the proper term is) from that date. If you have been told that you're 9 weeks from a scan then I'd pretty much say that you're 9 weeks. I guess it doesn't always fit in with the dates of your lmp because it depends on when the egg became 'mature'.

HTH :-)

Ellie9 Wed 05-Jan-11 20:02:25

Hi everyone,

I am a bit confused about this as well so it was good reading the posts, thank you.

I have another question which I am sure is obvious but I have no idea! Can you get pregnant when you are not ovulating? Silly I know but I am trying to work out date of conception etc. I am (according to measurement of scan 12 + 6.


violetwellies Wed 05-Jan-11 20:02:33

So - and now Im really confused - and I didnt think I was til I read all this..

My period was two weeks late, (so I would at that point be six weeks pregnant as am normally regular).

I then bled for six weeks, it stopped on the Monday and I went to the GP as I still felt like a bag of crap. (so I should have been 12 weeks pregnant just then).
The Gp said - your pregnant but I think your having a MC, (WHAT Im far too old for all that shock ) so had a scan -and bloods before a D&C, and lo there was a bean, 9 weeks plus 2 days said the sonographer - so was I 9+2 or was I 12 ? got my 20 week scan tomorrow so I guess it could be all change by then...

tebbles Wed 05-Jan-11 20:09:05


You would have be 9+2 like the sonographer said.

Think of it this way they would compare the development of your baby with all other babies conceived in the 'typical' timeframe. Your baby would fit the measurement and devlopment of a baby of 9+2.

Double check with the sonographer tomorrow!

ShowOfHands Wed 05-Jan-11 20:51:24

violetwellies, you just ovulated late I would imagine.

Ellie, can you get pregnant when not ovulating? I'm not sure I understand the question. Ovulation is the release of an egg from your ovary. No ovulation, no egg. No egg, no baby.

Unless you mean can you get pregnant if you have sex when you're not ovulating? Technically then it's a yes as the sperm can live inside you for up to about 5 days and wait around for ovulation.

Ellie9 Wed 05-Jan-11 21:02:55

thanks very much showofhands!

mummytobeagainxx Wed 25-Jan-12 13:14:49

im confused on this thread to as much as you lol at my ultrasound i measured 11 weeks 5 days it had put my last lmp at 16th july although it was 11th july so im guessing i ovulated later? at my scan though i measured crl at 11 weeks 5 days.. did i conceive then?? x

cATTYCAT125 Mon 09-May-16 18:13:17

On the 23rd of January I had a one night stand with a guy then again on the 31st of Jan, I'm 15 weeks pregnant, scan said i conceived on Jan 21st, can anyone tell me which guy it would be more so than the other ??

LaraG13 Wed 11-May-16 07:10:34

Hi catty, you might get more responses if you start up a new thread. Few questions to try to work out which guy is most likely to be the dad - What was the date of the first day of your last period before you got pregnant? do you normally have a standard cycle around 28 days? And were you exactly 15 weeks pregnant on 9/5?

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