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Pregnancy

Talk about every stage of pregnancy, from early symptoms to preparing for birth.

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Original poster

Confused over calculating dates....

14 replies

cosysocks · 19/05/2012 11:57

Had a scan yesterday and I have been put as 5+3.
The thing that is confusing me is these blinking two weeks that don't really exist but somehow get put onto your pregnancy.
Had a good chat with the midwife about it yesterday, my LMP was 27th March for 3 days. DH and I SWI on April 10th therefore when IPOAS on 23rd April I was just under 2 weeks pregnant.
Therefore I am 5 +3 as calculated by scan & midwife. Where then does this 2 weeks come into it? Every online calculator says 7 weeks... but the embryo is growing at a rate of actual weeks.
Can anyone help make this make sense? When does this magic two weeks get put onto your pregnancy? seems so confusing!

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MrsPaynie · 19/05/2012 12:01

Your pregnancy is dated from the date of your last period. So by the time your next period is due you are 4 weeks pregnant, however only 2 weeks from conception. The 40 weeks of pregnancy is counted from the first day of you LMP.

IvanaNap · 19/05/2012 12:04

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Original poster

cosysocks · 19/05/2012 12:07

It just makes NO sense! Why why why is it taken from LMP? it's sort of making the progress of embryo 2 weeks late before it has even started.

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IvanaNap · 19/05/2012 12:14

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DaisyMaisyJessicaEmily · 19/05/2012 12:17

because people generally don't know when they ovulated, but generally do know when their period was. simple :)

ItsMyLastOne · 19/05/2012 15:03

I thought it was to do with when the egg is maturing before it leaves your ovary. Iirc, 15 or so eggs mature, and then the one that is most viable is released. I had assumed that accounts for the time difference. But I don't know. Maybe it is just because many women don't know when they ovulated as Daisy said.

ItsMyLastOne · 19/05/2012 15:06

I just re-read your OP and I'm confused. Did they actually say at the scan you're 5+3? Because they would normally (if not always) go with the standard way of dating, which would put you at 7+ weeks.
I had an early scan at what I believe was 8+6 and the sonographer didn't give me a date but said it measured at 8+ weeks, so was going by the LMP dating rather than from ovulation iyswim.

Original poster

cosysocks · 19/05/2012 16:39

Itsmy yes they gave me 5+3 but prior to that I thought I was 7 weeks so bit

OP posts:

MrsPaynie · 19/05/2012 17:44

Cosysocks, you may have just ovulated later than you thought, or baby was too small to get an accurate measurement. You will get a more accurate dating at your next scan.

I've had early scans with all three of my pregnancies and all have put me a week later than I thought, but then by the time I had got to 12 weeks they all caught up and measured the same as my dates.

ItsMyLastOne · 19/05/2012 18:01

Oh that's strange. Have you seen a mw yet? Maybe it's worth asking about it and whether it's of any concern that there's a 2 week difference between your dates and the scan dating. But it probably is to do with the inaccuracy of measuring the baby when it's so small.
Did you watch him/her finding the measurement? Did it seem like they took much time over it? When I had mine she just kind of clicked vaguely at one end and vaguely at the other end, all very quick and at one end it was nowhere near the end of the baby iyswim. She said it measured 8+ weeks but I reckon if she'd done it properly it would have been more like 9+ weeks.
She did say that on an early scan they only check that everything looks normal and healthy (ie heart beat present and no obvious reason for bleeding in my case) and don't worry too much about dates until later on.

Spinity · 19/05/2012 18:44

It is confusing but does it really matter in the end? I always go by scan date, if they say you are x weeks then that seems more accurate than period/ovulation to me as they are actual measurements.

This time at my 12 week scan I was told I am more pregnant than I could possibly be by at least a week. Your due date is only a guideline after all (although it is of course taken as gospel and I will be very miffed should this baby be overdue) :)

ragged · 19/05/2012 19:21

It is a very stupid system, that's why!!
I think they should date pregnancy from first day of missed period. Would be just as "reliable" & would have at least equal validity.

Original poster

cosysocks · 19/05/2012 21:17

Hmm thinking about it she was very quick taking measurements. Got another scan on 28th so maybe the dates may have changed by then. I'm not worried by it, but it just seems so confusing with the added two weeks that appear.
I agree ragged if everyone sang from same hymn sheet as it were it would be much clearer.

OP posts:

ItsMyLastOne · 19/05/2012 21:45

Spinity there are two reasons it could be important. The first, most obvious one, is that the baby isn't developing as it should. The second is that it can affect how your birth goes. So if you go into labour at 36 weeks by their dates but 37 by your own they won't allow you to have a mw led birth as the baby is considered premature. Similarly if you hadn't gone into labour by 42 weeks by their dates but you were 41 weeks by your own dates then they'd be wanting to induce you but it wouldn't necessarily be done if they had the correct dates.

I'm sure that all will get cleared up for the OP at the next scan so it's probably not an issue here, but it can be for some women.

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