The main problem is that during pregnancy your hormone levels skyrocket. And when the pregnancy ends (for whatever reason), those hormones drop, sometimes at differing rates.
Your main form of estrogen, estradiol (E2), can increase 100 times normal levels during pregnancy. Once the pregnancy ends, the E2 levels drop to normal within 24 hours. Your progesterone increases 300 times and returns to normal within about a week. And the weakest (and thought to be the most protective) form of estrogen, estriol (E3) increases 1000 times normal levels.
Other hormones-- like cortisol and thyroid--may drop below normal before getting back in the groove after the pregnancy ends.
In my friend's case, her doctor had been supplementing her progesterone using bioidentical (chemically identical to the hormone our bodies make naturally) suppositories that deliver the hormone directly to the womb. This is often done to help a woman hold onto a pregnancy until the placenta begins to produce enough progesterone on its own to sustain the pregnancy.
So although she may not yet have developed the highest hormone levels of a full-term pregnancy, she nevertheless had a surging mix of hormones that took a nosedive when she lost the baby.
As with any post partum dysfunction, the elegant solution is to get those hormones back to normal. But that can take weeks or even months. What can a doctor do to help this along? Perhaps the best strategy might be to test the woman's hormones regularly and then supplement the ones needed to at least reestablish the normal proportions as the levels slowly find their way back down to normal.
In the absence of such a labor-intensive (and test-intensive) strategy, you can try taking SAMe. This is an over the counter (OTC) supplement you can find in most health food stores. I first heard about it from a midwives' group I spoke to. Since then I have seen research that confirms what the midwives told me: SAMe takes effect faster than conventional antidepressants (which take a while to build up in your system) and works just as well...with fewer side effects.
And if you know someone who is now pregnant and might be at risk for post partum depression, suggest that she start now taking high quality omega 3 oil supplements (fish/krill oil, or flax) throughout the pregnancy. These have been shown to protect against PPD for up to 8 months after delivery.
There are answers out there, perhaps more than we ever realized. We just have to ask the right questions.