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BMI Centile thing . . . don't understand! She's underweight!

(25 Posts)
WhoseGotMyEyebrows Fri 29-Jul-11 10:52:19

Can someone explain this to me. You know when the school measure and weigh your child and then tell you their BMI and if they are over/underweight . . .

Well mine's come back as healthy, which is great, but completely unexpected as my DC has always been at the bottom (and sometimes off the bottom) of the chart. It says that her BMI centile is 10 which puts her in the healthy range, but in her red book (she is weighed frequently) she is on 0.4 (bottom line).

Is it a different chart they use?

I know they take height into consideration so I wonder if it's that, although her height comes 2 lines above her weight. She is generally a small and very skinny child.

Confused. If my dd isn't too thin then what is!

(These conversations usually result in someone looking up an adult BMI calculator on line and children have their own I believe. Just to save the conversation going that way.)

bran Fri 29-Jul-11 10:57:11

Message withdrawn at poster's request.

RubberDuck Fri 29-Jul-11 10:58:33

BMI suggests her height and weight are in proportion with each other, the centile chart is a comparison between her and the general population - at her age, most children are taller and heavier. They're two different measurements and you need both for the big picture.

WhoseGotMyEyebrows Fri 29-Jul-11 11:05:20

So is the centile being 10 mean, that it is 10 in comparison to her height. Or 10 compared to the average for her age, population IYSWIM! No I've even confused myself!

ImeldaM Fri 29-Jul-11 11:11:11

Being in the 10th centile means that 90% of children her age will be taller and heavier than your DD.

Not sure why it is different in your red book though

ImeldaM Fri 29-Jul-11 11:19:10

Looked at DS red book, centiles are not about what height and weight are 'healthy', they are in reference to general population.

BMI is about what is healthy weight for height, not about comparisons with others.

So your DD is very small & light, especially compared to others her age, but not unhealthily skinny IYSWIM.

WhoseGotMyEyebrows Fri 29-Jul-11 12:10:32

That's what I mean though, that in her red book she is on the 0.4th and in the letter she's on the 10th. That's quite a big difference so wondered if it were worked out the same. I understand how they do it in the red book but wondered if the school measuring people work it out the same.

I'm wondering it there's been a mistake. Until very recently she was on build up drinks as she was off the bottom of the chart and was seeing the hospital on a regular basis.

ImeldaM So your saying that the BMI result bears to resemblance to the red book result? So 10 BMI is not the same as 10 Centile? In the letter it says BMI Centile which confuses it.

WhoseGotMyEyebrows Fri 29-Jul-11 12:11:08

bears NO resemblance!

LilyBolero Fri 29-Jul-11 12:14:10

The red books is purely a height chart and then a weight chart. If she is on the 0.4th centile, then 99.6% of children weigh more than she does.

However, if you have a very tall child, you would expect them to weigh more, without being unhealthy. So BMI is a calculation taking height AND weight into consideration, and that is why the centile result is different. 10th centile for BMI is within the healthy range.

nocake Fri 29-Jul-11 12:19:20

BMI is the ratio of weight to height. Roughly speaking it's how fat or thin you are.

The BMI Centile is showing where your child's BMI it compared to the general population. If she's on the 10 centile then 90% of children her age will have a BMI higher than hers.

The centile in the red book is based on weight alone so if she's on the 0.4 centile in the red book then 99.6% of children her age are heavier than she is.

nocake Fri 29-Jul-11 12:22:06

I'm not sure why they give you a BMI centile as BMI is an absolute figure. For adults a BMI of less than 20 is considered underweight and over 25 is considered overweight. It's irrelevant how you compare to the population as a whole.

WhoseGotMyEyebrows Fri 29-Jul-11 12:27:24

So some of you are saying that the BMI (not red book) shows just a figure based on wieght vs height, and some are saying it's compared to the general public?

LilyBolero Fri 29-Jul-11 12:32:41

The BMI figure is a figure, based on a calculation. A BMI CENTILE is compared to the population of children of that age.

LilyBolero Fri 29-Jul-11 12:33:15

Ds2 had his letter through recently, and though he is fairly small and light (20-25th centile?), his BMI centile was 50th.

WhoseGotMyEyebrows Fri 29-Jul-11 12:40:40

LilyBolero So that means that the figure should be the same as the one in the red book doesn't it? And it's not by quite a bit.

bruffin Fri 29-Jul-11 12:45:25

"So some of you are saying that the BMI (not red book) shows just a figure based on wieght vs height, and some are saying it's compared to the general public?"

It's both! ratio of weight versus height compared to others born that date.

Lilybolero - DD's BMI centile was 50% as well but, she was on 75% centile for height and i can't remember for her weight, but she was tall and thin but curvy (yr6 weigh in)

LawrieMarlow Fri 29-Jul-11 12:45:26

The red book is just for one part at once isn't it - so either height or weight. So she is 0.4 centile for weight and 9th(?) for height (I forget what the centile lines are).

BMI takes both into account. And not all children will have the same exact distribution of height and weight (ie not all on same centiles) so I think her BMI centile is likely to be different.

I may be talking twaddle of course.

WhoseGotMyEyebrows Fri 29-Jul-11 12:48:03

Yes Lawrie 0.4 for weight and just under the 9th for height.

RubberDuck Fri 29-Jul-11 14:10:45

It's two different measurements. BMI is calculated by a ratio of height and weight to see if you're in proportion. BMI CENTILE is a representation of how many of the population in that age range is heavier or lighter than you.

So her BMI is 10, but 99.6% of other children her age have a BMI greater than 10.

RubberDuck Fri 29-Jul-11 14:12:57

(You can also get height centiles and weight centiles which may be what is confusing you. The centile part is the indicator that it's a representation of a percentage, not that actual measurement).

WhoseGotMyEyebrows Fri 29-Jul-11 14:13:28

That makes sense RubberDuck

bruffin Fri 29-Jul-11 14:31:23

"So her BMI is 10, but 99.6% of other children her age have a BMI greater than 10"

Think OP is referring to weight chart in red book not BMI. The letters that get sent out refer to BMI centiles not actually BMI.

So 99.6% of children will be heavier than op's child,

but her BMI puts her on the 10th centile so 90% of children will have a higher bmi that she does.

WhoseGotMyEyebrows Fri 29-Jul-11 15:00:16

Buffin Right. OK. Think I get it now.

{walks away with brain ache}

LilyBolero Fri 29-Jul-11 17:59:40

yy, the BMI won't be the same as the red book.

jenniec79 Fri 29-Jul-11 18:02:36

There are 2 sets of centile charts (like red book) too - older ones are based on USA data for FF DCs. Newer ones relate better to BF babies - although it evens out after weaning, the BF babies tend to be lighter than FF, so check you're on the right chart!

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