Why do people some women get depressed after having a baby?

Possible theory: Estrogen levels are high in pregnancy and estrogen appears to inhibit a chemical called monoamine oxidase. Monoamine oxidase normally breaks down serotonin, Dopamine and norepinephrine. After delivery, estrogen levels rapidly decrease, increasing the activity of monoamine oxidase which in turn leads to more rapid breakdown of serotonin, etc. Postpartum depression is serious and needs treatment.