My 18 week old son weighs 13 pounds 11 ounces and in 13th percentile. i only breastfeed. should i switch to formula? height is 26 & 83rd percentile

Hard to know: A kid whose weight is behind their other growth parameters can have many issues involved. Poor feeding is one & BF more frequently to boost the milk supply may be one way to help. Supplementing briefly with formula sometimes boosts a poor feeders energy enough for them to feed better/stimulate production. There are also medical reasons for poor gain which the supervising doc should sort out.