It's thought to be caused by a combination of the fluctuating hormone levels associated with giving birth, the major transition to parenthood (or to having more than one child), and other life stresses that accompany pregnancy, childbirth, and parenting. If you have other major stressors affecting your life at the same time, or if you've been prone to depression in the past, you may be particularly vulnerable to postpartum depression. Symptoms can include feeling sad and hopeless; crying often; withdrawing from friends and family; eating or sleeping too little or too much; feeling worthless or guilty; and even being afraid of hurting yourself or the baby.

Many new mothers -- as many as 80%, states the National Mental Health Association -- experience the "baby blues" right after delivery, and these relatively mild symptoms (mood swings, crying spells, irritability) can go away within a few days to a few weeks. Treatment isn't necessarily needed, but support can be invaluable. Try seeking out  new-mom support groups through the hospital or birthing center where you delivered.

If the feelings linger, become severe, or if you have a family history of depression, it's important to get treatment for postpartum depression. Two great sources of help are Postpartum Support International (http://www.postpartum.net/), which has a helpline at (800) 944-4PPD ((800) 944-4773), and the National Mental Health Association (http://www.nmha.org/). You can search for a mental health professional near you on its web site, or call them at (800) 969-6642.