Every baby tracker shows growth percentiles. "Your daughter is in the 72nd percentile for weight." It looks like a lookup — find the row for her age, compare, print a number.
It isn't. And the ways it goes wrong are interesting enough to be worth writing down.
I ended up implementing this properly for a baby journal app, and published the result as who-growth-standards — MIT, zero dependencies, all the WHO tables bundled. This post is the reasoning behind it.
The problem with "just look it up"
The WHO Child Growth Standards don't publish percentiles directly. They publish three numbers per age per sex: L, M and S.
