BACKGROUND: Women in pregnancy and postpartum have an increased vulnerability to develop an affective disorder. Affective disorders in pregnancy are associated with an increased risk of prematurity, dysmaturity (foetal weight below the 10th percentile as determined by ultrasound) and the development of postpartum depressive disorder. Untreated affective disorders and their complications may also result in considerable costs. Recent meta-analyses showed that interventions during pregnancy are less effective than postpartum interventions probably because of high attrition due to the barriers pregnant women experience with attending sessions outside their homes. An internet-based self-help intervention may overcome these barriers as it can be followed at home, and also in one's own time. Such internet interventions showed to be effective for decreasing affective symptoms in general.This randomised clinical trial examines whether an internet-based self-help intervention is effective in the reduction of affective symptoms in pregnancy and postpartum and results in an improvement of the perinatal outcome. We will also determine the cost-effectiveness of the intervention.