Perinatal depression affects 10-15% of women postpartum and has a recurrence rate of 40%. Women who develop perinatal depression might be particularly susceptible to the rapid and large changes in sex steroid hormones, particularly estradiol, across pregnancy to postpartum. This trial aims 1) to evaluate the preventive effect of transdermal estradiol treatment in the immediate postpartum on depressive episodes in a subgroup of women at high-risk for perinatal depression, and 2) to determine if a set of biomarker gene transcripts can identify this subgroup and thus form the basis for future personalised prevention or treatment. The MAMA Trial is a double-blind, 1:1 randomised, placebo-controlled trial. The trial involves maternity wards at three university hospitals in the Capital Region of Denmark. Women who are singleton pregnant in the third trimester with a prior history of perinatal depression are eligible to participate. Participants will be randomised to either estradiol patches (200 μg per day) or placebo patches for three weeks starting immediately postpartum. The primary statistical analysis will be performed based on the intention-to-treat principle. A sample size of 220 will provide the trial with 80% power (alpha 0.05, beta 0.2) to detect a reduction in postpartum depression of 50% and to tolerate a drop-out of around 20%.