Using the WKB approximation for the evaluation of penetrabilities at stellar energies, the D(d, p)T and D(d,n)3He total cross-sections are analysed in the energy range 0–100 keV (c.m.). The s and p wave contributions are extracted in each of these reactions. The branching ratios calculated from this partial wave decomposition are in good agreement with the experimental values deduced from muon catalyzed fusion and with those calculated from R-matrix theory. The screening effect is studied and the U0 electron screening potential discussed.