Objective: To test the hypothesis that disturbances in plasma melatonin distinguish pregnant and postpartum patients with major depression (DP) from matched healthy comparison (HC) women.

Method: In 25 pregnant (15 HC, 10 DP) and 24 postpartum (11 HC, 13 DP) women, we measured plasma melatonin every 30 minutes from 18:00-11:00 h in dim light (< 30 lux).

Results: Pregnant DP had significantly lower (p = .01), whereas postpartum DP had higher plasma melatonin values (p = .02) in the early morning hours, compared with matched HC women. Pregnant (but not postpartum) women with a personal or family history of depression, regardless of current diagnosis, had significantly earlier melatonin synthesis offset (p = .002) and standard offset (p = .007) than women without such a history. In pregnant DP vs. HC women, sleep onset was significantly delayed relative to melatonin onset (p = .01), and synthesis offset (p = .002) and standard melatonin offset (p = .004) were significantly advanced relative to sleep onset.

Conclusions: Plasma melatonin concentrations during sleep in the morning hours, were lower in depressed pregnant, but elevated in depressed postpartum women, compared with HC women. Melatonin timing measures were advanced in pregnant women with a personal or family history of depression. It is not currently known whether such differences in the timing and amount of melatonin secretion are causal in DP.