Scan dates an centiles - I don't understand!

[Darcourse]

This is going to be convoluted I think. I'm really confused about how the sonographer dates a pregnancy and where the centiles come into play.

I had my 12 week scan at exactly 12 weeks by my dates (I know my dates as IVF pregnancy) but I was dated 12 weeks & 3 days BUT my babies were under the 50th centile. How can that be? How can they accurately say a baby is 12 weeks x days when there is a variable centile chart?

I had another scan today at 16+2 by my dates (16+5) by their dates, and babies are down to 25th centile, yet on screen the dates that we're coming up matched my dates exactly. I'm really confused?

And will any if this matter if they need to bring the babies out early? Could those 3 days be vital at say 34/35 weeks?

They can tell most accurately before 12 weeks because most babies follow the same growth line until then (obviously some variation but minor). After that it's more hit and miss. Three days isn't really going to make any difference.

Thanks Sanch. What I don't understand though is how is 12 weeks 3 days on 40th centile different from a baby at 12 weeks exactly on, say, the 60th centile?

I assume they take an average of the measurements... So my foetuses have both had short legs (but so does all my husband's family so no no suprise there). That means the scan dates them but then some bits are above and below the percentile. I also think there are key measurements which determine the date and then others which determine the percentile.. So maybe the head measurement is for dates and the limbs and tummy for percentile? (I don't know this.. I'm just guessing).

Also a friend of mine had an ivf baby and they moved her date if conception. She said it was really tricky when they kept talking about how far along she was when she clearly wasn't.. It was only a week or so but she was having NONE of it! She was on a massive mission to go into labour before they induced her 'early'.

Bonkers!!

I have no idea of the answe Mr to your main question, but those three days will only ever be vital at the extremes of gestational age: at 23/24 weeks, and at 42 weeks. But going into labour at those extremes is so unlikely, I wouldn't worry about it for now.
Best of luck with your pregnancy b

Thanks - that's reassuring Dr!

I'm not a professional but believe Katy is right here. I think certain parts of the body i.e. lungs and heart will be taken accurately and other variables will differ. Baby might have a head on the 70th centile, but legs on the 20th. When I had my 20 week scan it shows you a little thing at the bottom that has head, stomach etc and a line like this --- and a little mark somewhere across it. So if mine read like -^--- for example it would probably be around 60% on the centile chart if that makes sense?

Then they take the 'average' for your baby with all the measurements considered and give you a centile.

Unless it's very high or very low I'd not be concerned.

RE: a few days difference like Doctor said unless you give birth at 23 weeks and therefore the lungs haven't got the capacity to breathe independently yet then you don't have a problem. There is no massive difference say from 34+4 and 35 weeks, for example. The only real difference could possibly be a little extra weight (and I mean a little, 100g or something small like that) and the amount of hair they have.