Low levels of DHA fatty acid have been found to correlate with PPD. Baby uses up alot of your stores to buid her brain. Make sure you replenish them with either a good (molecularly distilled is best to avoid heavy metals) fish oil supplement (I use Nordic Naturals), and/or fish, eggs etc. Plant sources are almost nonexistant, making vegetarians more vulnerable to DHA and EPA deficiency. Omega 3 fatty acids from walnuts, linseed etc are important, too, but you and baby do need the DHA and EPA!
To balance the postpartum hormones, Susun Weed also has this herbal tea recipe