My DS is 12.5 months old and isn't quite double his birth weight. He was born on the 75th centile and after tracking the 25th centile for a while, then the 9th, he has now fallen to the 2nd. He's also 2nd centile for height. Health visitors never seem concerned about it. I'm average sized and DH is taller than average. Would you be worried if this were your baby?