Big changes take place in your body and brain during pregnancy. Certain hormones are in play during pregnancy and after the baby is born, big changes take place again. Hormones affect your brain, and especially if you have a history of MI, it can cause depression. It usually has nothing to do with the baby, or being a mother or family changes, but any or all of those things can add stress which can negatively affect you. If you or someone you know thinks they have PPD, seek help asap.
PPD is not very well understood by anyone. I have been thru it and can't possibly describe how I felt. My baby was very much wanted and loved, and I was in a stable loving marriage. There was nothing at all negative associated with my being pregnant. However, I had been on meds before pregnancy and was off during pregnancy, so I think that is why I ended up in bad shape.
But, it was the weirdest I've ever felt and little is really known about what is going on. I went into treatment and was treated as normal depression, and luckily it worked, but some women may need other kinds of help.