A report by the Royal College of Nursing says only one trust out of 28 is implementing proper procedures to ensure new mothers get the help they need.
The RCN describes post-natal depression as a complex and distressing problem which can affect the baby, the father and other children as well as the mother.
Its survey shows only one trust, Fife Primary Care Trust, was complying with Department of Health guidelines issued earlier this year.
These advocated setting up an integrated system of care which would ensure help for the woman up to a year after birth.
The report's author, Jim Marr, said: "Some of the trusts seem not to have got their act together quickly enough following the recommendations from the government.
"Post-natal depression can affect 100 to 150 women per thousand births every year in Scotland - that's potentially up to 6,000 women.
"There's still an expectation in our society that the birth of a baby is a joyous event and everything is rosy. For some women, that is not the reality.
"There needs to be an acknowledgement that post-natal depression is a real problem for women in Scotland so that women don't feel embarrassed to say they are depressed.
"Many worry that an admission of depression will lead to the baby being taken away from them."
The RCN is calling for all trusts to adopt the guidelines and offer each mother-to-be a trained health care professional who can provide support.
It wants trusts to acknowledge that in a high number of cases domestic violence begins during pregnancy.
The college also says childbirth education classes should focus on preparation for parenthood and relationship issues and not just the physical aspects of labour and delivery.