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I never tried calculating it. But dd1 had a lot less than dd2. Up to 6 months they both weighed much the same. The hv just said dd2 must have a fast metabolism . I think as long as their nappies are wet and they are putting on weight no need to worry
It's fine - if she's keeping to her line then she's putting on weight. All babies are totally different. If you got ten adults the same size/age and gave them all the same amount of food they wouldn't eat the same way.
My DD was exactly the same - was always a bit below what she 'should' have been drinking. She was just naturally small and skinny. She even dropped a percentile but she ate regularly and had chubby cheeks and LOADS of energy. She still is skinny at 18 months but perfectly healthy and growing very tall!
I wouldn't worry. It's only a guide. She'll only drink what her body needs, you can't make dc drink more ... She'll also be getting some nutrients from her food. Mine never drank the recommended but have turned into healthy 19, 13, 9 and 7 year olds