Independent market analysts Datamonitor say that the postnatal condition is "under-treated" in the UK.
They call for hospitals to make more effort to screen new mothers for the condition.
It is thought that full postnatal depression affects as many as 15% of women shortly after birth.
Many more have a short period of feeling low after the birth, often described as the "baby blues".
However, postnatal depression may emerge, either as a worsening of these feelings, or several weeks after the birth, and require anti-depressant treatment or counselling to resolve it.
Long term
If not tackled professionally, the depression could resolve itself in time - but may turn into a chronic and long-term illness.
Of those diagnosed, say Datamonitor, only a small proportion - perhaps one in three - are prescribed antidepressant drugs.
There are fears among mothers that such drugs may be passed to babies via breast milk, although trial data suggests this is not the case.
Nick Alcock, from Datamonitor, said: "Postnatal depression is a serious medical condition which has not been taken seriously by all sections of the healthcare community.
"It can lead to a life of chronic depression if patients remain untreated, which can impact not just that patient, but the development of the infant and family unit as a whole.
"Healthcare systems need to put into place guidelines which ensure that screening for postnatal depression is more regular, and that treatment, when appropriate, is for a minimum of six months as opposed to six weeks."