Healthy breastfed infants tend to grow more rapidly than formula-fed infants in the first 2-3 months of life and less rapidly from 3 to 12 months. All growth charts available at this time include data from infants who were not exclusively breastfed for the first 6 months (includes formula-fed infants and those starting solids before the recommended 6 months). Because many doctors are not aware of this, they see the baby dropping in percentiles on the growth chart and often come to the faulty conclusion that the baby is not growing adequately.
At this point they often recommend that the mother (unnecessarily) supplement with formula or solids, and sometimes recommend that they stop breastfeeding altogether. Even if mom realizes that her baby is perfectly healthy and doesn't follow these unnecessary recommendations, she ends up worrying for no reason (and moms don't need anything extra to worry about!).