According to a Swedish study first time mothers who gave birth after the age of 35 were twice as likely as younger mothers to develop a mental illness after the birth.

These mothers also stood double the risk of suffering from postpartum psychosis, a mental illness where the person is unable to distinguish between reality and imagination and could involve a danger to the newborn child, needing to be treated with anti-psychotic drugs.

The study was conducted by researchers at the Karolinska Institute in Stockholm to assess the risk factors associated with psychotic illness after childbirth. Published by the Public Library of Science medical journal, the researchers said, "The risk of developing psychosis during the first 90 days (after childbirth) increased with age."

The researchers studied data from nearly 750,000 first time mothers in Sweden who gave birth between 1983 and 2000. They found that women over 35 who gave birth for the first time were 2.4 times more likely to develop postpartum psychosis than those who were younger than 19.

Although as many as 80 % of women experience postpartum blues or mild depression this is usually short lived. Only one in 1,000 women suffer from psychosis in the initial months after giving birth which is a far more serious condition.

Often involving delusions, hallucinations, severe eating or sleeping disturbances and suicidal tendencies, postpartum psychosis can be dangerous for both the mother and the newborn child as the chances of self inflicted harm and suicide are greater as are chances of harm to the baby.

Immediate medical attention, as well as the administration of anti-psychotic drugs and hospitalisation are called for in such cases. The researchers said that although in a majority of such cases the women have prior psychotic history in almost 50 % of the cases that were seen "without prior psychotic hospitalisation."

Factors such as smoking, not living with the baby's father had no impact on the woman developing postpartum psychosis. However, suffering from diabetes and giving birth to babies with higher birth weights was seen to reduce the risk.

The researchers said the study findings highlight the need to need to carefully monitor women in the first month post delivery. They said it is not clear why the risk of psychosis is the greatest then but the extreme hormonal fluctuations that the women experience makes some more susceptible.

The research is published by the Public Library of Science medical journal.