O90.6 is postpartum mood disturbance, not postpartum depression.
The World Health Organization classifies postpartum depression as one of three things: Postpartum blues, postpartum non-psychotic depression, and postpartum psychotic depression.
Postpartum blues onset within day 3 or 4, can last hours to days, and requires no treatment other than reassurance. This is O90.6.
Postpartum depression can occur any time within 12 months of childbirth, lasts weeks to months, and requires treatment.
Postpartum depression with psychosis usually occurs within 2 weeks of childbirth, can last weeks to months, and usually requires hospitalization.
There are clinical criteria for postpartum depression (same as for major depression).