Conventional estimates of cold deuterium-deuterium (D-D) fusion rate and branching ratio may not be reliable, since they are based on an extrapolation of the reaction cross sections at higher energies (≳ 4 keV) to lower energies where no direct measurements exist. Recent results of indirect measurements of the cross section indicate that the extrapolation method may not be valid at low energies. Direct measurements of the D-D fusion reaction cross section at low energies are suggested.