By the modularity theorem, an elliptic curve $E$ over $\mathbf Q$ of conductor $N$ admits a surjection $\varphi$ from the modular curve $X_0(N)$. The Manin constant $c$ of such a modular parametrization of $E$ is the integer that scales the differential associated to the normalized newform on $\Gamma_0(N)$ determined by the isogeny class of $E$ to the $\varphi$-pullback of a Néron differential of $E$. For optimal $\varphi$ Manin conjectured his constant to be $1$, and we show that in general it divides $\operatorname{deg}(\varphi)$ under mild assumptions at the primes $2$ and $3$. This gives new restrictions on the primes that could divide the Manin constant. The talk is based on joint work with Michael Neururer and Abhishek Saha.