A 2009 research study showed that there is a correlation between perinatal depression and low levels of certain key nutrients. The study showed that there are links between low levels of folate, vitamin B12, calcium, iron, selenium, and zinc, and mood in general. Post-Pregnancy-Depression can be attributed to the depletion of those key nutrients.
“Perinatal depression: prevalence, [...]

Care2.com had an article titled “A better way to Improve Mood“. Here are a couple of interesting article highlights:
““We’re facing a bad-mood epidemic”
“Of course, neither amino acids nor antidepressants work for everyone, but a 1991 head-to-head comparison published in the medical journalPsychopathology found that the amino acid 5-hydroxytryptophan (5-HTP) actually outperformed the antidepressant Luvox in [...]