The Pythagoras number of fields of transcendence degree $1$ over $\mathbb{Q}$

We show that any sum of squares in a field of transcendence degree $1$ over $\mathbb{Q}$ is a sum of $5$ squares, answering a question of Pop and Pfister. We deduce this result from a representation theorem, in $k(C)$, for quadratic forms of rank $\ge 5$ with coefficients in $k$, where $C$ is a curve over a number field $k$.