Chiral effective field theory predicts a specific charge symmetry violating amplitude for pion production. This term is shown to provide the dominant contribution to the forward-backward asymmetry in the angular distribution for the reaction pn → dπ 0 , for reasonable values of the mass difference between down and up quarks, δm N . Using a value δm N ≈ 3 MeV leads to a prediction of a 10 standard deviation effect for a TRIUMF experiment.