All over the world mothers are given numbers as their baby grows. The numbers are from ‘growth charts’ showing how a baby is developing in comparison to others. Seven month old Baby Arlo has particularly big numbers, so much so that his parents are worried he’s one of the biggest babies in America. But where do these numbers come from? Is it an average? Why do they measure a baby’s head? Reporter Jordan Dunbar sets out to find out how we get these baby numbers and just how big Baby Arlo is.

Presenter: Tim Harford and Jordan Dunbar