On average, breastfed babies weigh less than formula fed babies. Most pediatricians use the formula fed weight chart as a way to track weight and length, regardless of how the baby is fed. This can mean that your breastfed baby may be on the lower percentages for his/her age group and still be very healthy.
The WHO (The World Health Organization) has a breastfed growth standard chart for babies. This chart puts into account that breastfed babies do tend to grow a bit slower vs formula fed babies. It also shows the average growth pattern, of other babies, from birth on up.
So when you are told your child is in the 10 % or less (like we were) try not to panic. Check the WHO and see what it says. Plus, you do not have to supplement, with formula, there are other ways to help your baby rise in the charts. But that will be another post.