What is postpartum depression?
Postpartum depression is a mood disorder that occurs within the first year postpartum, and typically within the second month postpartum.
The symptoms include sadness and hopelessness, shame ("I'm the worst mother ever..." or "why is everyone so much better at this than me?"), irritability, anxiety, crying “for no reason,” oversleeping or difficulty sleeping, difficulty concentrating or remembering details, or making decisions, anger or rage, loss of interest in previously enjoyable activities, physical problems including headaches, stomach problems and muscle pain, eating too little or too much, withdrawing from friends and family, difficulty feeling an emotional attachment to the baby, doubting one’s ability to care for a baby (and feeling like your baby thinks you're a terrible parent), and thinking about harming the baby.
Risk factors for postpartum depression are biological, social, psychological: previous depression and depression in pregnancy, previous trauma, traumatic birth, relationship instability, lack of family and community support, poverty, financial instability and job insecurity, health problems with the baby, a stressful life event during pregnancy, and alcohol or drug abuse. It is compounded by loss of sleep, difficulties with breastfeeding, and other challenges around caring for a new baby. I would add that an additional risk factor is cultural. When there are expectations that we should just "bounce up" and take care of everything--new baby, partner, kids, home, maybe back at work within a few weeks, and so on--or that we should be grateful for just having a healthy baby and forgetting whatever else we might have been through, then these can make it harder to ask for help.
It might not be postpartum depression...
Thyroid hormone imbalances are common in the postpartum, and commonly misdiagnosed as postpartum depression. I recommend doing lab work to check your thyroid, including TSH, thyroid antibodies, free T4 and free T3. It is not enough to only check TSH, so insist that your provider run a full panel.
The "postpartum blues" is a transient period of emotional ups and downs, sensitivity, irritability, and fatigue that occurs with the hormonal transition postpartum, typically between days 2-4, and is finished by the time postpartum depression shows up.