Postnatal psychosis or PNP, is a mental illness, which involves a complete break with reality. Although correctly termed as a postnatal stress disorder or postpartum depressive reaction, postnatal psychosis is different from postpartum depression. The majority of PNP occurs within the first 2–4 weeks after childbirth with a classic 10–14 day meltdown, likely caused by the radical hormonal changes combined with neurotransmitter overactivity. When correctly diagnosed at the earliest signs and immediately treated with anti-psychotic medication, the illness is recoverable within a few weeks. If undiagnosed, even for just a few days, it can take the woman months to recover. In cases of PNP, the sufferer is often unaware that she is unwell.[10]

Symptoms of PNP can include:
Feelings of being ordered by God or a power outside of oneself to do things one would not normally do, like harming oneself or the baby[2]
Feelings of intense confusion or agitation[2]
Seeing or hearing things that others don't[2]
Extreme highs or extreme lows of energy or mood[2]
Inability to take care of the baby[2]
Experiencing thoughts and feelings as being out of one's control[2]
Memory lapses (periods of confusion similar to amnesia)
Random or uncontrollable anxiety attacks
Unintelligible speech or communication

Psychosis can also take place in combination with an underlying psychiatric disorder, such as bipolar affective disorder, schizophrenia, or undiagnosed depression. In some women, a postnatal psychosis is the only psychotic episode they will ever experience, but, for others, it is just the first indication of a psychiatric disorder. Only 1 to 2 women per 1,000 births (.1% to .2% of births) develop postnatal psychosis.[11] It is a rare condition, and often treatable. However, much media coverage of postpartum depression has focused on psychosis, especially following the Andrea Yates case. Whilst postnatal/puerperal psychosis is a serious psychiatric illness, the risks of a mother suffering this illness harming her baby are low: infanticide rates are estimated at 4%, and suicide rates in postnatal/puerperal psychosis are estimated at 5%.

PNP is also known as "postnatal stress disorder", because the patients are notably under emotional stress and exhibits unusual behavioral patterns not seen before their pregnancy or postpartum event.