Participants were postpartum Women with Major Depressive Disorder (n=42) who received online BA supported in 30-minute telephone sessions by a mental health worker. Depression symptoms were assessed at each session and number of sessions completed were recorded by the online program. Therapist records were rated for client stressful life event and therapist concrete focus. A quadratic pattern provided the best fit with the data; a cubic pattern was a poor fit. Sudden gains, but not depression spikes, predicted lower depression scores at 17-week outcome. Women who had higher baseline social functioning, did not experience a stressful life event during therapy, and completed more online modules, but not more telephone sessions, were more likely to have a sudden gain. A concrete therapist focus was associated with sudden gains. These results extend research on trajectories of change and sudden gains to an online BA treatment and to postpartum depression, and suggest important client and therapist factors associated with sudden gains.