Autumns sister Summer SGA still, should I start worrying?

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Hi all, quick note. Autumns sister Summer, 4 mos, is still not

catching up from her IUGR and is now reached 10 lbs and 21.5 inches,

(born 4 13 and 17.5 inches so good consistant growth so far) amazingly

enough is identical to Autumn's growth at this age except her head size

is not on the charts and is proportionate to weight/height. she has

no signs of catch up at all although she looks a little chunky and

Autumn did not at all, I wonder why, maybe Autumns larger head weighed

more and Summer's weight is fat? Does that sound strange? I am just

worried because the older Autumn gets the worse she seems to be and

Summer although someone different, is walking the same path. Right now

her weight/length is good! But she is so little and it is not

somethign seen in my family except Autumn. All my kids have dropped

further from their birth place on the charts at 6 mos or so and that is

coming soon for Summer. Any input?